They will probably always at least retain their value. A gem mint set even for the non holos nowadays is just simply out of reach for a lot of collectors so a 9 set will be the reasonable alternative. There will always be people who want to collect but can’t go all in for 10s.
If pure investment stick to the big 3. The other cards are probably safe to hold value for the next few years, but if you’re looking for a return I don’t see it as the safest way to go. It’s not bad, but I would rather take my chances at a unlimited big 3 vs a 1st ed non holo.
I don’t forsee them going down in value if that’s what you mean by relatively safe. If you’re going for a collection and want the cards but don’t want them to go down in value you should be good. If you’re looking for a pure investment with the best return for your money I would go elsewhere.
If you want the best return on investment long term, I feel the PSA 10 options are the best way to go in terms of price potential in the future.
That being said, considering the PSA 9 prices they are a really good entry point to buy a larger amount now if you can sit on them for a while. If you had to sell them in a short time (year or so) you probably would break even after shipping