If it was me doing it (spoiler alert - it's not) I'd present as much as possible of 5.5 as 'options' rather than actual changes. So when you get a Ranger class feature at level 6, for instance, you'd get to choose from a list, including the original PHB feature, or the Tasha's alternate feature, or whatever. Give the monk a d10 hit dice and fix some of the more feeble subclasses while you're at it if you must, and I suspect nobody will complain.
Spell changes being back compatible isn't as big a deal as it was back in 3.x, simply because the Concentration limit means that there's fewer module encounters or PC builds that rely on the complicated interaction of seven different stacking buff spells. Will it probably screw over those people who've built some finely optimised Booming Blade Hexblade/Paladin monstrosity or similar? Well, yeah, inevitably. When you stretch the rules to the edge case, and the rules change to fix up the edges, you're gonna get cut. And frankly, that doesn't bother me at all.