I'm sorry not to add anything relevant after what you just said, but I had to respond to say that it's actually a really good idea, and I'd like to see that. Apart from that, you're forgetting that they can just make it so only Pokemon captured in Galar and onward can be transfered, in future game, thus removing compatibility with old CT spreading like Toxic. I don't think that's a good play, but that's one play.Maybe not the place to talk about it, but I find it extremely frustrating that BDSP Thanos snapped the compatability of TM06 Toxic. I know that it still remains compatible on all Pokémon (except VGC formats), but it still comes across as a lazy choice instead of trying to balance it with the tools they already have present in game.
If they wanted to remove it almost completely to desensitize degenerate stall plays, I don't see why they wouldn't make it more of a high risk high reward move with a sinister downside. If they wanted to make it "make sense" by having only a handful of relevant Poison/Poison-adjacent Pokémon learn it, then I think it's a bit too late for that.
My take on how the move should work takes both of those points in mind.
I think Toxic should be re-made available to all Pokémon who could previously learn it, but it should be brought back down to it's previous 85% accuracy (or even down to 70%). Furthermore, if the user's Toxic should miss the opponent, I think it should badly Poison the user. This sounds awful, and would almost completely decensitize the usage for Pokémon like Landorus-T or Hippowdon, but it would offer some unique interactions.
Firstly, it would be a nice mirror to how Toxic always hits when used by a Poison type. Otherwise, Steel types would still be unaffected, albeit the lower accuracy would balance the issue a bit more.
The poster children of stall and serial Toxic usersChansey and Blissey wouldn't suffer too badly since they have Natural Cure as their primary ability and would be able to switch out to remove the harm done to themselves. Other Pokémon with Natural Cure like Altaria who often use Toxic would also share this unique interaction and be more favorable as Toxic spreaders in their respective tiers.
Mentioning those 3 prior Pokémon reminded me that they all have access to either Heal Bell or Aromatherapy, as do other great Pokémon like Clefable or Mew. Clerics could become a bit more of a niche in consequence to players wanting to use Toxic on mons like Lando and Hippo.
Clefable amongst others also has Magic Guard to avoid damage completely.
Another fun interaction would be Poison Heal Breloom or Gliscor holding an item other than Toxic Orb to benefit from either poisoning the target, getting poisoned by missing and therefore healing passively, and also holding a viable/non-wasted hold item like Lefties or HDB.
Marvel Scale Pokémon like Milotic and (Eviolite) Dragonair could also benefit from accidentally being poisoned, though to a lesser extent than Breloom and Gliscor.
None of these interactions make the move better than it currently is, and that isn't the point. The side effect of badly poisoning yourself will almost never be good, but as a way to desensitize the move as a whole, I think it works and it still provides room for some Pokémon to use the move either completely or almost completely unhindered.