I want to create some new feats that grant a single cantrip and a 3rd-level spell. The feats will have the prerequisite that the PC must be a spellcaster with access to 3rd-level spells already, and although it uses their own spellcasting class's spell save DC, they can only cast it "once per long rest", not with their spell slots (I'm not sure if they could use spell slots if the spell is already on their spell list - obviously the main point of this is to gain spells not usually available to you, so in practice this won't happen).
The obvious comparison is with the Magic Initiate feat, except that Magic Initiate allows you to pick any two cantrips and any 1st level spell from any class's spell list (although all must come from the same spell list), whereas the spells for my feats will be a specific cantrip and spell. I'm hoping this lack of versatility (and the fact that non-spellcasters can't get it) will bring it in line with Magic Initiate despite granting a 3rd-level spell.
I plan on giving themes to these feats. Specifically, the "Desert Master" feat will give mold earth and wall of sand, and the "Ocean Master" feat will give shape water and wall of water (I may come up with better names at some point, but these'll do for now).
Note that, at the moment, these are the only two feats I am considering, so any other 3rd-level spells being granted by similar homebrew feats of this style are outside of the scope of this question, which is specifically about the "Desert/Ocean Master" feats above. Should I come up with more feats along these lines, I will try to balance any other 3rd-level spells I choose against these two spells by myself at the time.
Are these two feats balanced? Does giving specific cantrips (and non-damaging cantrips at that) and 3rd-level "wall of X" spells counterbalance the fact that I'm granting a 3rd-level spell via a feat?