Re: IC: low magic, slightly gritty D&D 5E
Most of the best healing magic is pretty low level though. So unless all casting is done away with for PC's, there is still good healing options up to 3rd level spells. And if 5th level spells are let in you've got Mass Cure Wounds, Mass Healing Word, Raise Dead, Revivify, Prayer of Healing, Healing Spirit, Greater Restoration on the table potentially.
5th level is pretty powerful without being too over the top IMO. I think you let PC casters into the game personally, with the understanding that casting is limited (in whatever way is finalized).
Maybe it could even be something like a normal caster progression up to 9th or 10th level (5th level slots), and then allow for the PC's to be those "legendary people" once/if the game ever gets there, but limited in some way. I.e. give them something like a Warlock's Mystic Arcanum but you dole out the spells, and you take out all the spells that are totally game breaking to you.
So when they hit 11th level, they get a single 6th level slot, and so on, until they have 9th level slots. But they can only use those slots to use their 1-5th level spells at greater effectiveness/power, UNLESS the PC finds an Epic spell stela or something that has a single 6th (7th,8th,9th) level spell they can actually *know*/learn. Then it's RP/decisions as to which PC gets to learn it, and whether they even want to commit that spell (whatever it is) to their repertoire as their only 6th (or higher) option.
That way you limit the availability of high level spells to what you want to allow into the game, but the PC's get to be those legendary casters (eventually) who can push the bounds of magic beyond what most people have been able to do for centuries (6/7/8/9 spell slots and maybe spells even). Whose spells are more powerful than thought possible (using those slots to cast lower level spells), etc.
In this scenario you could also filter out certain 5th level spells that you might think are troublesome to the game you want to run. Ex: Teleportation circle, Animate Objects (tiny), plus... maybe Steel Wind Strike and Cloudkill...?
I'd love to get in there and help think through this stuff with you.