I am a veteran player just coming back to Dragonflight. Ideally I will play Horde, any Realm recommendations where gold is more valuable? The idea is to be able to pay my monthly only with gold.

I guess that means staying away from more populated servers like Area 52 or Illidan, since gold tends to be worth less where it's easier to get. Or am I wrong?

Doesn't seem to be always true in regards to realm population affecting prices. Prices vary quite a bit regardless of supply and demand, at least from what I've seen on EU.

Just as a side note, you can get gametime (60 days) or b.net balance gifted for fg, here: https://forums.d2jsp.org/forum.php?f=386