Personally, my first impression is that your defence has a fair bit of passivity to it… You have 2 healers (rigard and Grazul) and a reviver. Means that you only have 2 damage dealers.
I would suggest reducing the amount of healers personally…
My long term suggestion might be to look at running:
Alby → Kong → Richard → Poseidon → Kingston
Kong is an unusual suggestion but is better than azlar imo… Better speed, similar level of damage and a buff afterwards… Slow isn’t great for anything but tank and even then isn’t fantastic…
I chose double green because that’s the strongest options you have available.
Doubling the yellows isn’t great unless you have a purple to split them (as you said, grimble ain’t a great option)
The second issue I can forsee is that both Poseidon and Kingston are fighters so may compete for emblems. If this is a worry you can switch Onatel in for Poseidon but…
Alby and Kingston both need to be in the D. They’re two of the best heroes in the game.
Rigard out for Kingston is the clear first upgrade.
Onatel is a second obvious ‘level up ASAP’ as she’s excellent - though Poseidon is very good too. Personally I’d rather have Onatel on defence most of the time and she’s probably a better tank option long term than Richard… but I actually think Richard is underrated - especially if you can snag his costume.
Grimble is a bit of a flyer - id hold out til you have mats to ascend a second 5star purple and if no other 5 star comes along before, go for grimble. Thing about him is he may be ‘before his time’ - it’s possible the game may see a rush of minion based characters and Grimble will then be very good to have ready. But it’s only speculation at this point.
You have two tank options - Richard and Onatel. Justice is a lower desire ability option. Rich and Ona have strong rolls offensively too, so having them both up is good.
Both make solid flanks to a tank too, so you could swap them interchangeably
But if you have FishKing already leveled, there is less push needed for Onatel - just level her at a normal pace.
Kingston is your priority (and Lianna too for offence bc Kingston+Lianna = dead enemy)
Grazul isn’t terrible on the other flank from Ona/Pos. None of your reds are obviously stronger. You could elevate Azlar and go for a riskier defence - if he fires you probably win; but he’s so fragile and sow he’ll likely only rarely fire.