I often filter files, to not download everything.

I guess this is the reason.

As long as I don't have ongoing download I want to finish, can I safely delete all those?

I dont know for sure, but be careful not to delete the crust pieces.

If you only chose files for download once, it is highly likely that all that data is needed to serve complete pieces that are partially occupied by chosen files. Big piece size in big torrents can result in significant growth of that directory.

If you often changed set of active files, downloaded some, deleted others, etc., Tixati used to leak parts that were not needed by any present file any more. It is supposed to be fixed since a couple of versions ago. I am not sure that it automatically checks and cleans old torrents. Starting a manual hash check could be needed for suspicious torrents.

Yes, best done with Tixati off (If Tixati needs anything in there it can redownload it anyway)