I have nothing ready for Ganyu, much like when I got Hu Tao so it's probably going to be at most 300 dollars to get her started. Shenhe, at least for now is being called a support. I want to use the Mona I lucked out on getting, Venti is a personal fave too and I also like Ayaka so I feel like Ganyu is the best investment for now, because I know I want to have the other 5* that go in her best teams.
If Shenhe remains top tier in support or sub-dps, I don't have Cryo dps worthy of her support, and thus I'd be wasting a lot of in game time just waiting for banners when I could be building characters that will lead to my eventual dual Abyss teams.
Ah I see what the problem for you is now. First things first, I'll say this, get whatever character that you feel that you like the most always.
Secondly, when it comes to cryo dps, almost all of them are viable, are they as good as ayaka/ganyu? Probably not. But that's fine, you can beat every content in the game with every unit, so units like kaeya, rosaria and chong while not as good can still do enough, someone like shenhe is just future proofing and can be used in any team, supports are flexible, dps arent.
Generally speaking ganyu's freeze teams consist mostly of 5*s so that's another thing to consider.
One more thing, Ganyu was also seen as a support before her release, and just ended up bonkers but you're right, those times characters weren't as reserved as they are now that's why bennet is the best character in the game and xiangling is broken aoe pyro carry.
Overall I'd say you're justified in going for ganyu, she's a good pick and needs no investment (whale bow) to be good, and if she ever falls behind, so will every other dps, if you're fine with the charge shot playstyle.
