I have direct thread and I notice when doing range work I need to retighten every so often (4-6 shots) as it does start to loosen ever so slightly. I put witness lines so I can tell visually. My intended use is primarily for hunting so this why I did direct thread to keep weight and length to a minimum.
If that's the case you're not tightening your can enough, I've never had a DT can come loose that was properly tightened even shooting matches.
Agreed. If you tighten them adequately they will not come loose during shooting or hunting. Trust me, you aren't strong enough to damage your can or your barrel by hand tightening your suppressor.
In regards to the OPs ask, volume is the key to suppression. The larger the volume of the can, the better the suppression. If weight doesn't matter at all, then you will want the biggest suppressor you can get. One with a replaceable end cap that you can match with the caliber of the round is even better.