The fact that we sometimes receive negotiation offers for our players who are not for sale shows that the release clause set will be very useful to the seller and will also facilitate the transfer between clubs.

I agree. Especially for inactive clubs who leave their talented players to rot in their clubs

Would setting the release clause be optional? Would this forbid bidding for those who don't have release clause set (and who aren't on transfer / auction list) or would it forbid bidding less than the set release clause for payers who have release clause or both or something else?

I think you can achieve something similar by massaging the bidder, negotiating and/or rejecting an offer.

As for the inactive clubs, I'd rather have an option to delete a club once the person decides they won't play the game anymore and release all the player for auction sale rather than just have all good players taken away and let them be a punching bags soon after. How would you determine if the club is inactive? If they haven't logged-in for a few months? Also, I don't know what happens when their contracts expire nowadays , I assume they are released for auction sale, but don't know.