The 61254 tires are a decent fit for the rear/shoulder wheels. They don't fall off, but they're not snug, and slip loose after rolling a bit. So I may need to try and check if poster tack or floor polish or super glue would help them stay in place. Of course I'm leery about experimenting with even more substance reactions.
Also, the thing is I forgot that the wheels were different sizes. So now I'm trying to find some 50861 tires locally for the front/knee wheels, and also have a handful of the bigger ones I don't need.
For something like this, I’d recommend some scotch tape. It can deform as needed, is removable, thickens more than the liquids and is accessible.Just cut it carefully to keep it concealed.
Thanks to unluckiness' tip, I was able to get the rear/shoulder tires to seat well after a few layers of scotch tape. I may go back and add another layer to Beachcomber, whose testing I kind of rushed.
Even better, I gambled on a cheapo bootleg Lego bike whose description seemed to have measurements that matched the 50861. I say "seemed" because I had no way of knowing whether the numbers referred to the tire or the whole Lego bike. (Logically, they made more sense for the tires, but you can genuinely never tell with these listings.)
"Well, that explains why I couldn't take this out this morning. Remind me to have a talk with Rattrap about what we can and can't barter away for Energon goodies around here."
Happily, as it turns out, these tires are a perfectly snug fit for the front/knee wheels, zero modifications. Putting them on was a joy after the slight disappointment of the previous pair.
I don't think Lego type equivalents have been found for Sandstorm (and his tires are reportedly much harder to remove anyway). Legacy G1 Op/Magnus I think I saw a post and part number for, but I haven't gotten around to those yet. Wanted to see this pair through first, and so far so good. Ordering another bike for just its tires, now. Although idk what I'll do with what will be two awkwardly-sized bikes missing their tires, afterward.

I mean, something in me is healed a little bit by seeing... this, but it's undercut somewhat by Gigastorm's new whip not having any tires.
Of course, the silly thing about all this is that since I don't have access to a legitimate source for ordering official Lego parts, all of these are bootlegs from who knows where and might have similar plasticizer problems anyway. And even the legit ones might not be all that different in composition, and it's too soon to really tell. But it was a cheap and easy enough experiment that it seemed worthwhile; I could at least feel that I tried something.