yes, exactly. Qubes OS is already complicated enough for new users, don’t try a non standard setup that you would not feel comfortable debugging on your own 
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Haha, thanks, guys. But this isn’t a complicated dracut-module. I’ve been using Kicksecure and Ubuntu with grub-live for over a year. But the Qubes have their own specifics with trim, and I got confused. But your recommendations helped me! Big thanks!
So, trim works by default in btrfs. In lvm qubes, varlibqubes also have the file driver, so trim works by default only in vm-pool. For varlibqubes, there are only 2 solutions: a new installation of templates or duplicating templates in vm-pool and cloning from vm-pool to varlibqubes when the templates in varlibqubes take up all the free space in dom0