@Noble_Weasel
Starting point is you only need to squeeze into Diamon to open the chest, it doesn’t matter what level you were while filling it, just what level you are when you OPEN it (so even if completed in Platinum if you open it in Diamond you get Diamond rewards).
At first it will be frustrating as you barely nudge your way into Diamond, gradually you will have higher high scores and higher low scores until eventually you stay in Diamond overnight.
You can get there and hold with some 4* in the line-up. I was still using Sabina as my healer when I finally stabilized in Diamond and Boril tanking (still tanking although now he is very effective at +10) , At the time only one 5* was maxed before I stabilized in Diamond, Sartana.
So a good combo of 4* with mostly leveled 5* can get you to the door and even a peek inside, that will boost the mats collection which will speed the leveling in a nice virtuous circle.
I LOVE Boril as a tank and this allows you the time to see if you pull another blue hero that you prefer to Thorne, if not, he is actually fairly tough with the recent buff. Not “THE BEST” but better than before and he hits hard. He can be flank to Onatel but I feel that leaves a side open without riposte and it just that little bit easier for the attacker.
Onatel - she is a good tank. She is not the one that strikes the most fear but and Onatel/Boril team will certainly keep you close to Diamond.
IF you take Onatel to final ascension it will be a long time before you start working on Poseidon so the question is, will Onatel work well enough at 3-70 while bringing Poseidon to 3-70 or would you prefer Onatel at 4-xx and Poseidon at 1-1?
Getting to Diamond is all about attacking and which combo will help you win a greater percentage of your attacking raids?
My opinion is that Onatel @ 3-70 + Boril 4-70 will keep you close enough to open the chest in Diamond and you will win more with Poseidon as an attacking option once he is in 3-xx ascension.
Your Defense Team:
Danza (?) - Sabina - Onatel - Boril - Gormek
I don’t like the color stacking on D but Sabina is better than Melendor based on your emblems, Grimm isn’t great on D, Danza maybe your best option or you could add Tibby (if you used Tibby, Boril tanks, Onatel flanks), I like him and his two good specials are great specials and if he is last Ewok standing then it is a bit all or nothing.
Other Option:
Sabina - Onatel - Boril - Tibby - Gormek
Either of those should keep you close, second option may be the better option.
The other reason to advocate for Onatel @ 3-70 plus Poseidon, class challenges and war, you need a deeper bench for those.