my first big project - BMS - is it possible?

Yeah, okay, those might work well.

But check the Farnell price for Belgium!

5,14 Euros for Qty 100+

compared to 3.59 Euros for Qty 1 relay.

Now, since this is for a motorbike/scooter, the SSR might make much better sense.
Why not keep searching, see if some other more reasonably priced alternative can be found.