Let's rip off the band-aid first: Your roster could use a lot of work. I don't think you should be focusing on getting your heroes to 40 right now. Quite frankly, most of your roster should be retired (and not just because of all the 
/ 
faces). This is probably going to take a lot of time and work.
Now for the good news. This is pretty normal for someone in your situation who hasn't really taken their roster seriously yet. Additionally, you don't have a lot of heroes who are irredeemably bad. Most of them have at least a little going for them, which is a good sign.
I would very highly recommend you put at least three points into the Argon's Finest talent in the Questing tree. This talent lets you hire new heroes at a higher level, and three points gives you level 10 fresh hires. This way you can see their first two skills immediately, which helps a lot when rerolling a large amount of heroes.
Elemental coverage is your biggest issue right now. This isn't bad, but it could be a lot better. Aim to always have a minimum of three heroes per element as your first major target, then branch out to three heroes / two archetypes each. This gives you a lot of flexibility for lategame content. I'd recommend you check out the Sample Roster over on ST Central (in the TL,DR Heroes section) if you'd like an example of what this looks like.
I'd recommend staying away from the T7 classes, Spellblades, and Samurai for now. T7s are crazy expensive to hire and are largely weaker than their T6 counterparts. Samurai / Spellblades are easily the two most difficult classes to build in the game. I would avoid all five of these classes, at least until you have a better handle on your roster.
When phasing out old heroes, do them two or three at a time. Don't move on to the next set until the new heroes you've replaced them with are ones you know you're going to keep. It's OK if these aren't "perfect" heroes, having skillsets good enough for them to stick around for a long time is your target. Ignore the "reroll until two gold skills" or "reroll until two 
skills" thing you've probably heard a bunch - it's pretty bad advice in my opinion.
I'm going to use some terms you may not have heard before and probably say some things that seem odd or may not make sense. If you have any questions, please ask! I know this is going to be a lot of information.
(yes, this is all the introduction to the actual breakdown. that gif i posted earlier is a meme on the stc discord for a reason.)
late quest
warm helm
tribal citrus
