To me ideal elf would be:
1. Elven immunities
2. Elven trance
3. Darkvision 60ft
4. Proficiency+Expertise in perception
5. one extra cantrip or +5ft move speed or +60ft darkvison, depending on subspecies.
then add +2/+1/+1 ASI
Then add background/culture;
+3 skills
+3 tools/languages/weapons
+1 feat
This is the Elf character concept that you want to build.
Using the Mythal design above, maybe something like this.
ELF [character sheet]
Creature Type: Fey Human
Size: Medium [6 feet tall]
Speed: 30 feet
Lifespan: [Age 20] Eternal youth (but dont get killed)
Trance
Fey Ancestry (Charm Resistance)
Cantrip
Moon Mythal (Darkvision, Perception)
So far, the main difference is, you want to swap out the slot-1 and slot-2 spells. But you arent getting anything in return for it, so you might as well keep these. They are kind of nice, even tho they arrive later at higher levels.
Now the background design space:
ability scores: +2, +1
2 skills
1 tool
1 feat
Note, every character has a language, normally but not necessarily "Common". Additionally, the species grants a choice of language (whatever language the species community chooses to speak and not necessarily the indigenous language), and the background grants a choice of language (whatever language the individual makes an effort to learn).
Mainly, you are swapping the slot-1 and slot-2 spells for an extra +1 ability score improvement to a third ability. Balancewise, that roughly fair, and as a DM I would happily allow the swap. But since a tertiary floating score bump is kinda flavorless, I personally prefer it be unofficial.
Meanwhile, I wouldnt worry about the third skill, since the class will be adding even more skills. And if it is imperative to the character concept to get that extra skill, it is worth spending the feat to obtain it.
Thinking about your design preferences:
The slot-1 and slot-2 spells are together worth a feat. (The spells come very late in levels, but since the player can choose from any spell list, that is probably for the best.) It is possible to have the slot spells be a Mythal: Innate Caster Mythal.
Then the Elf design would be: Trance, Charm Resistance, Cantrip, plus a choice of two Mythals.
Meanwhile Innate Caster and Moon would be the two default Mythals (for players who dont want to make the effort to choose the Mythals).