lets say i have 0.5 ETH and want to convert it all into BTC.
Is there a possibility to make the ETH amount fixed and with the changing price the amount of BTC adjusts instead?
No. You can only fix the price in BTC.
This feature was planned to be added, but there were talks recently that this might help against certain scams in Bisq when fiat is being used, so I don’t think it is planed to be added, at least not for fiat trades. I guess it can be added for altcoins however, but I devs are probably busy with other work currently.
bummer 
but i can understand that you can do only so much at the same time.