PokeGuardian/PokeBeach usually get the information out early enough. I’d also add Pokeboon.
The next easily accessible thing to do is follow the social media of the various Pokemon affiliated sites like twitter.com/Pokemon_cojp or their actual website www.pokemon.co.jp/info/. Even e4 will mention most of the promos if you venture out of the price discussion and bulk of the general threads.
But as for what cards to buy as an investment that’s another story.
Yeah any card in the 150-350 range is cheap af. Once things get above 500 and starts impacting your wallet a little more, you gotta be a little less frivelrous with your spending. Once things get in the $2,000+ range, you gotta be frugal and calculated on what you buy. Or at least, that’s my mindset.