I’ve been collecting Psyduck for a variety of reasons and it’s been fun to see all of the variants, artworks, sets, etc. And not really that pricey compared to some of the more popular Pokemon.
Then I remembered Psyduck is in every Tropical Wind/Wave/Beach & Champions Festival cards. Every. Single. One. 
I think it’s fun having a range of difficulty with species collecting. I collect Magikarp, and I love that while I can get a lot of the cards for literal cents, there is still Snap Magikarp at the mountain top. Having that challenge has kept me engaged in the hobby. If I was able to get every Magikarp card in a few days, it definitely wouldn’t be as exciting for me.
Ngl idk why anyone collects any other Pokemon when Bidoof 1HKOs them all
San Diego Comic Con Piplup is staring a hole through my spreadsheet as we speak. (For those unaware, there are only 5 copies in existence making it worth a very pretty penny, let alone the possibility of actually seeing one in the wild.)
Then you get into cameos (so many illustration contest cameos. So many), other languages besides English and Japanese, and it gets to be quite a hunt!
However, to answer the question of the thread, I collect Piplup line because I love them- simple as that. Seeing all of the little (and big) blue penguins in my binder and just flipping through the pages makes me incredibly happy. They really do spark joy, and that, and its core, is what collecting is to me- curating a group of items that kindles that spark of joy or nostalgia or love or what have you.
Collecting for the sake of completion is great and all, but if you’re not having fun with it, what’s the point? I don’t think collecting solely to have a complete collection of whatever you’re going for is a good way to go about it. If you don’t have a passion for the collection itself, then hunting down items won’t be rewarding at all.
For example, someone could hand me the rarest sports card in existence and it wouldn’t mean a thing to me because I don’t care about them, but hand that same card to a person with a passion and it’s like night and day.
If you want to collect Umbreon, then collect Umbreon! Just because you can’t have a complete collection right now doesn’t mean you shouldn’t do what you can ever, yknow?
I think @wooltchi hit it on the head, although Mantyke can be ‘reasonably’ expensive, since a lot of people know it only has a single English artwork and want it for that reason. Bonsly, Budew, and some others like Archen/Archeops for example are therefore cheaper/easier to complete.
I collect Pikachu (in all languages up to Feb. 27th, 2021, and in just English and one of each exclusive artwork after that date) and Seviper (in all languages and variations).
I started collecting Pikachu mid-2015. The main reason I collect it: I figured it would keep me busy for a while, since there are so many of them. Welp… that was an understatement 
I currently own over 1750 unique Pikachu TCG cards and own the second largest collection in the world a.f.a.i.k. From the Pikachu cards released before Feb. 27th, 2021 (25th Pokémon Day), I’m missing just 10. Four of those just never pop up for sale, and the other six are simply out of my price range (and also barely pop up for sale tbch…) The reason I stopped from Feb. 27th, 2021 onward: I simply couldn’t afford to continue at the same pace anymore. In 2019 alone, 233 Pikachu TCG cards were released. That’s about a Pikachu every 1.5 days on average. And since the pandemic, cards have also become more expensive on average. Not to mention Pikachu isn’t my only collection goal.
As for Seviper: unlike Pikachu, it’s my favorite Pokémon. I’ve always liked snakes as a kid, and although I like some of the other snake Pokémon as well (e.g. Ekans, Arbok, Onix, Serperior, etc.), Seviper’s design was on a whole other level. Apart from the three Simplified Chinese Seviper cards, for which I’m still trying to find a good source, that collection is 100% complete. Although I’m currently focusing on non-TCG products. Here some pictures for those interested.
Greetz,
Quuador
I’m working on a PSA 9 Gyarados English Master set. Gyarados has always been my favorite since I was a kid so this has been a ton of fun! And the best part is aside from the WOTC era cards and a handful of others, it’s been very affordable. I mean hell I’ve purchased a decent chunk of this set for $20-30 per card (sometimes less).
This is my set on the PSA registry. I’m at 63% not counting a few cards that are in-transit (more than that if you only count the TCG cards). I’ve always enjoyed set collecting and this has been my favorite collection goal by far! (side note: if anyone has anything I’m missing on there in a PSA 9 give me a shout 
)
After I eventually complete this I’ll probably try for the true Master set. Well, except for the Snap Gyarados probably 
Shedinja only has 8 cards in English unfortunately, so now that means I must have the master set
Try not to let some big ticket cards keep you away from the Pokemon you like best! Watching the collection of your favorite grow is a whole different kind of good feeling
Umbreon is a tough one to collect, however it is important to realize that you dont need to get every single one! I would consider this, especially if its one you like!
Species/Trainer collecting has become a way for me to focus my collection, and it’s made things more fun. My first year or so back into collecting I collected bits here and there, whatever whimsy came to me, I’d get it. On top of that I opened a lot of booster packs cuz I missed the thrill of pulling stuff. Except, now packs and pull rates are so much more of a money pit. Pull rates are terrible compared to WOTC era, holos are now a dime a dozen, bulk price, etc.
Anyways, I realized I wasn’t getting what I wanted as much out of the hobby so I decided to narrow my focus to my favorites. I started with Raichu, Japanese Neo holos and collecting artists I liked in a binder which made me appreciate my collection a lot more!
I’ve since shifted my focus again to collect my favorite trainer (Karen) and team rocket trainer cards
As well as focus on my top favorite artist, Naoyo Kimura. I’ve had to come to grips with letting some Karen cameo cards go due to steep prices/insane rarity, much less trying to get more than one copy.
(Such as a certain e-series trophy with her silhouette on it and a certain Morty’s Hypno by Kimura from the TMB English half deck 
)
TLDR; collecting is tough, but collect what you love and collect what you can within your means. You’ll enjoy it a lot more 
I just recently got back into collecting and am starting small. I love the design of the Hoppip evolution line, so I’ve been collecting those pieces recently. One of each individual art for now and not max rarity and no cameos, but that may change later! And so far, the Hoppip line seems pretty affordable and easy to get into!
First, IMHO, Collections don’t have to be complete. They simply have to bring you joy.
Still, my pokemon are as follows:
Porygon (only difficult one is the players club)
Gastly
Xatu
I chose these because they ARE collectible, but also because I enjoyed them, and I like collections that can track with the history of the game, so they would HAVE to be gen 1 or 2. I also have no problem including expensive cards as proxies, and I think this is perfectly fine, depending on the level of OCD you have in yourself.
That Ursaring art is top tier though haha.