I’ve done a lot of mixing and matching with 5e and A5e in my own campaign, and I plan to incorporate some aspects from ToV, but I think it’s important to keep each character class, heritage/ancestry/species limited to their own source books. You can try a little blending (such as with subclasses/archetypes) but the DM should be ready to restrict or homebrew certain combos that may not mix well.
ToV classes will probably need to be more restricted to their own content, mainly for the reason that their subclass levels are standardized and not compatible with 5e.
I think they will largely play fine together, side by side, though you may run into level ranges where ToV characters gain features at different rates. Some of the ToV classes and feats also seem slightly stronger than their 2014 counterparts (but maybe more in line with 2024 - only speculating as I don’t have the book). I think A5e classes offer more versatility but should also play fine with ToV classes.