"Why not join the war? It’s in inevitable he’ll be forced into it given the empire’s animosity towards beastmen and his support of beastmen :/"
Inevitable? maybe.... The MC is not a murderpuppy, is really only interested in his Slow Life, and can handle any direct threats to him and his without breaking a sweat...
Our MC is an adventurer and as such reserves the right to make his own choices which commissions he wants to accept. And under which conditions.
He is not beholden to the empire, he has already trained the principal powers/leaders who are, rescued the imperial family on commission to reset the board, and him meddling with a war on one side any further will drastically reduce the chances of that Slow Life ever happening.
And no... His proto-harem is not a reason.. None of the ladies have displayed the spine to hook-line-and-sinker our MC into any kind of obligation in that respect.