Well, this was unexpected! Atm I have 4 Syno boxes, a 412+, 918+ and double RS3614RPXS+ with expansions (still waiting for me to start using them in my new place).
But the turn of events landed me one more unit. A brand new 718+ as a payment for services.
Who can say no to that? Right? :D. Well, now I have a new "toy" to play around with and plan is to turn both 718 and 918 into all flash storage and use RS3614 as my "slow" high capacity storage.
In a few months I'll revisit this and see what will be my decision.