Looking for feedback form people who have played more systems, have used the spell poing system from the DMG, or have ideas on what I am trying to do.
IME - Spellcasting systems that use a skill check/target number setup like you're proposing also typically include some kind of resource that depletes with casting (often the caster's health) to provide a reason why the caster isn't just always casting spells all of the time. I've also seen systems where missing the target number causes a hideous backlash that does horrible things the more you fumble the roll by, though I'm not a fan of punishing players for bad rolls personally. The only ones I can think of that don't have something like that tend to be superhero games where the spellcaster is in a world where everyone else has reliable powers they can access all of the time and mechanically spellcasters are exchanging reliability for versatility - I played with using something like that in D&D back in my youthful days, but I never could get the balance to work right.
I would also suggest if you want to keep a D&D like flavor on the system that you have some kind of way of preventing a 1st level character from casting 9th level spells - unless you like the idea of having access to Meteor Swarm and Power Word Kill, in which case knock yourself out. It's actually probably more of a problem with the healing spells - the spell slot restrictions keep healing spells to a certain level. Remove that restriction and you should expect everyone to be at full health pretty much all of the time unless you've given the spellcaster some kind of limited resource like mana or tied spellcasting to their health (which opens up weird questions about the caster using healing spells to heal themselves and whether there's a net gain or net loss there - a rabbit hole down which I also ventured in my misspent youth).
Finally: if you haven't looked at it you should check out the Ars Magica magic system. Pretty much the a number one go-to skillcheck spellcasting system in my book. Of course that game presumes that everyone will be a wizard so it isn't balanced at all for D&D play, but it's worth checking out for ideas.