Lots of good advice here - take it. You’ve already got the makings of a very strong team - LoL, King, Sarty, Victor… but it’s going to be 6 months down the road or so while you build up the mats to get there.
If you are going to spend $$ looking for Mats, DONT spend gems on the offers in the shop that include a tiny chance for them (trainers tools is on right now - don’t buy it) - wait til special offers come up that include a guaranteed 4* item.
The shop offers for loot tickets and trainers are a much better investment of limited resources.
DO make sure you have a strong set of 3s (at least 2 in each colour; and a strong set of 4s - this will let you compete in the 2x weekly tournaments faster and earn more mats; and also let you compete in the events that happen that limit hero usage by level - and offer good mats.
This is the most efficient way to build a team that will earn you more mats to build a better team fastest.
It’s not the “right” way to do it, but “my” way to level 3/4 stars is to pick a pair of good 4*s and a trio of 3 stars all in unique colours and level them up together. The three stars go fast and the four stars make you feel like you’re building power.
Once you get within 1 mat of having all resources for a 5 star - sub them into your building roster and take them to 3/70. You’ll either get the mat during the build or it will come along soon after.
Do one five star at a time until your three and fours are strong. Feed only colour to like-colour for the 20% bonus.
Rigard; BoldTusk; Kiril; Grimm; Caedemon; Danz; Gormek - all really strong four stars that you’ll use for a long time.
But don’t ignore the 3Stars… you need them too, and they build fast and their mats are easy to get.
In your base make sure you push to get at least one Training Camp to level 20. Ignore researching ALL of the colour specific levels inside the TCs. They are terribly inefficient.
Research lvls 1, 2, 11, 19, 20.
Once TC20 is up and running you can start letting RNG fill out holes in your five star bench with the likes of Lianna, Vivica; Richard etc. There are some strong season 1 heroes that you can have for free. Don’t ignore that.
Talk to ya in a year and you’ll be in diamond.