Features I am looking for:
- slow progression
- low to medium magic
- a good set of fantasy monsters
- not gritty
- not clunky
I haven't stated it particularily clear in my first post, but I want to run a fantasy campaign with elves, dwarves, goblins, dragons, evil necromancers and dumb ogres.
Level 3 to 10 is my favorite in dnd, or more specifically, 5 to 8. The math, complexity and resource management is just right.
Before you look into a completely new system, have you thought about simply house-ruling your favourite edition of D&D?
If you want to have a complete game with its own books so that you feel "safe", not pressured with working on rules yourself, it is understable, especially if your players are skeptic about house rules and trust published books better.
But reading your "wanted features" makes me think that you don't really need to change much to get them out of D&D.
1) Start from "low to medium magic". I think your already know your favourite level range so use that as a starting point, just assume for now that you'll simply play D&D (your favourite edition) starting from THAT level,
core only. Then you only need to check if some spells in that level range have problems for you (e.g. the usual polymorphs, teleports, fly...) and simply remove them from the game if they feel too high-magic.
2) Check the number progression across those levels. Just assume that your campaign will not go beyond that last level 8 or 10. Is this resulting progression "short" enough? E.g. in 3.x going from level 3 to 10 means that character BAB will increase max +7, and the max different between any two PCs will be +5. Is this "flat enough"? If it's not, shorten the range.
3) Once you know your max level according to the cap you want on magic and on numbers (including numbers
differences between PCs), don't worry too much about what happens after your level cap until you actually reach it.

But you can declare that after that level cap, levels stop progressing and (if you want) PCs get additional features one by one. For example you can use the E6 rules, except that instead of "6" you can put your chosen level cap.
4) Monsters: I think you already have nearly all iconic monsters before level 10 (remember that 3.x gives you plenty of low/mid-CR dragons). If some of them is missing, you can work on re-fluffing another monster from the appropriate CR range and swapping a couple of its features with the wanted features of the iconic monster. It's not a big work, and once again do it only when you need it!
5) I think grittiness depends mostly on equipment. If you level cap is low enough, even 3.x can sustain significant deviations from standard character wealth (but 3.x is absolutely not gritty so probably it's already ok for you!).
So all this essentially
doesn't really look like house-ruling much. You can just keep using the same rules for character creation, exploration, combat... those features you want don't require you to change them.
If you want
more levelling up
within the range (maybe you like the features of the level range, but not the fact that it's got only 5-7 levels?), you can dedicate some work into splitting up each level into smaller increments.
I'm suggesting all this because if I wanted the features you listed (none of which depend on the mechanics of combat or other challenges), I would not take the pains of learning a completely different game, I would look simply into adjusting the game I am most familar with.
