Reducing hit points across the board, bounded accuracy, changing saves, curating spells, making monsters scary – there are worthy reasons to explore these changes.
However...
...I think Magic Resistance is not the best way to go... especially when you're asserting that "Most spells aren't broken"!
When you have multiple weapon attacks, entirely missing with one isn't a terrible loss... compared to when you only have one spell and it entirely fails that loss is more keenly felt.
Additionally, Magic Resistance is a second point of failure beyond the spell attack / saving throw. Most weapon attacks only have one point of failure: the attack roll.
Not to get too into the weeds, but research was done suggesting that a success rate around 70-75% gives strong motivation to keep striving without falling into giving up. I can't remember if this was from Mihaly Csikszentmihályi's work on "flow", but I've seen game designers reference it in their blogs.
Anything like Magic Resistance ("no your spell does nothing") should be used verrrrry cautiously due to the impact it has on players. If your game system plays super fast (B/X or AD&D) so losing your turn doesn't mean waiting as long as you would in a 3e, 4e, or 5e game...that might be ok. In a 5e styled game? I wouldn't do it.