This next BP can be a breeze a first time for me. T-15/57 is so lowest priority and non-importance to me. If 72M4 is on rank 40, then it is totally flop and no go, if it would be a sink for pieces in shop - doubtful I spend coins on it. Which means i have high chance I stop on rank 30 without issues.
So i assume, If TR-85M1 is first tank to pick on BP rank 30 and Derivacya is shop-based piece basis, it is perfect awesome deal. Derivacya is fine and unique vehicle too. I like TR-85M1, at least with some footages it gives a deceiving impression of pretty modern, cool looking, agile tank (as for extensive T55 mod).