Is there a mod that stops the apartment landlord from spawning?
I want to create a new custom neighborhood without any NPC but I'd love to play with apartments.
I know there's a mod that allows you to rent residential lots (https://modthesims.info/download.php?t=171883) but this is not what I'm looking for.
I just need the landlord to not spawn or at least to not start interactions with my sims.

You probably can't prevent a landlord spawning, if you play any apartment lot.
You can probably use a mod to ban them from appearing on the lot (such as Twojeffs' SimBlender).
If you don't allow the landlord to appear at all, I don't think your sims will be able to rent the apartment in the first place.

That is what I was worrying about.
Even if I use the visitor controller I think my sims won't be able to rent apartments.
I was hoping someone made a mod that made the renting process "automatic".

Not that I can think of, sorry.