I’d almost max your first rainbow set of 4*s (including a 5* 3/70 if you have a very good reason, like being a top hero in the game or no other reasonable choices), and you’re doing that.
Early in my game, I kinda stopped working heroes when I got them to ~4/60, and just fed them the occasional trainer hero, because those last 10 levels don’t make a huge difference, and they are sooooooooo expensive!
So once you get them to that point, I’d focus on your second rainbow set, and take them also to ~4/60, stopping there for the same reason.
Do the 2nd rainbow set next, because then you can stack two of any color you want, and run raids, quests, S2 Hard levels at 2/1/1/1 or 2/2/1, which is a HUGE advantage.
If you have enough heroes you want to level, after that, then I’d seriously consider leveling up your next 4 rainbow sets (to 30 heroes) more-or-less in parallel.
First start taking sets to ~2/20, often where their special hits 8/8. These heroes are way more powerful for war attacks than unleveled heroes, and are VERY CHEAP to make.
Then start taking sets of your favorite ~2/20 heroes up to 3/60 until you have 30 heroes at that level.
Also, around the time you get 10 maxed 4* heroes (maybe a 5* 3/70 or two), you should be getting the mats and the resources to start taking 5* heroes into their final ascension. That’s a nice detour when you can do it.
And continue working on ascending your 3/60 heroes a set at a time 'till you have 25~30 heroes.
I’m just getting there now (30 4* / 5* heroes maxed with an additional silly number of 3* heroes maxed) after almost 9 months playing.
