I'm always curious how other people are doing with renting, so your title caught my eye for sure.
If I can give any advice, I'd look for cards that are cheap relative to what you can rent them for. Arcane Thug was fetching absurdly high rental income for how cheap it was to buy for a little bit. That cooled off, however.
Summoners usually do well, although they always cost a little more to buy. If you can get a couple leveled up summoners as you save for leveling them up to where you ultimately want them to play with, it can get you some decent DEC.
Also - be mindful of the league level caps. Usually cards at a league cap get more DEC/BCX than the level under. For quick reference:
Bronze: 3C; 2R; 2E; 1L
Silver: 5C; 4R; 3E; 2L