Well, YIKES!!

Looks like you have green (therefore viable) branches, and a BUNCH of ROOTS! So you're starting on the plus side. This will be basically like an "AI" response (but from a real human) because I'm just telling you everything I can think of/learned in 40+ years of "trying" to Garden.(!)

First, we are supposed to change out soil, every two or three years. I was about to chide you for using cactus mix, but turns out you're on the right track; they sell something called "palm/citrus/cactus mix." Who knew! You need a mix that holds water, but drains well. I also ran across something called "Lemon Grove" for planting/potting lemon trees, on Amazon. Amazon also has a cheaper one similar but half the price. You may be able to find local (go to a real Nursery, and expect to pay more if you buy it there rather than online or at BigBox store.)

I used to listen to a Nursery Guy on a radio call-in show, who advised using fertilizer (specific for lemon trees/citrus) at half-rate, twice as often. In your case I think I'd go by the label instructions and watch how it responds before making adjustments (reminds me of ppl in recipe comments who never make the recipe the way it was demonstrated, "I used nutmeg instead of cinnamon, and I hate bell peppers so j just put in an extra cucumber"🤭)

Full disclosure, your Meyer Lemon looks about like mine (potted/stuck out in on unloved flower bet in the south forty), which I've neglected for years, tho it does get watered (have *never* changed out the soil, (!)... I was sure it was a goner, then my DH came in with a couple little LEMONS from my neglected child! Now I'm all enthused to take all the advice above, and revive my baby for some payoff later on.

Anyway, I live in California, so cold isn't a huge problem. Occasionally gets to the teens, but usually no worse than low 20s briefly. I've never coddled it-- it just stays put all winter, and does it's best to stay alive. If you live where temps stay low for long, maybe just keep it in the garage, without expecting it to make leaves or lemons. Do give it water once in a while, for the roots' sake. Bring out to patio when freezing no longer a risk, and wait for new leaves to start. Fertilize after it's acclimated to outdoor air/wate

If you only get freezy cold occasionally, I think moving the pot to an outside heated-house wall, that gets morning sun but not north winds, AND covering it with plant covers (See amazon), should be enough to protect it and eliminate the shock of moving to new environment that causes leaf drop.

One more thing (sorry so long a reply)... you have enough green stem there that you could start a new Meyer Lemon. I'm not gonna try to tell you how, but urge you to watch a ton of You Tubes (search term "propagate Meyer Lemon tree")... Send the kids to Grandma's, and send your DH out Fishing! It'll be a Rabbit Hole day of watching/learning.

This is worth learning/doing, as I *think* I read where Meyer Lemon are no longer being produced. (Maybe just California?)

Good Luck-- to us both! 🍋