How to umount /var /usr safely on systemd without reboot

I have a Linux server on a VM which the reboot does work as powering off due to misconfiguration of a third part provider. I do not have access to VM configuration. The person who installed the system did a mess up with the storage and have irresponsibly mounted one point for each directory (/var, /home, /usr, etc...) leading them to be easely starved for some and empty for others. In order to fix that mess I am reorganizing the mount points, I was able to manage most of them by doing mount --bind / /mnt followed by rsync and then relaunching the process who use them after umount. The problem is the /var and /usr which is used by systemd init process itself. Would systemd-remount-fs does the trick? How could I permorm that? Would be a simple fstab edit followed by rsync be enough? Will it restart all the services? I know which points really does need separate partitions for my case, and it is not the case of /var and /usr at all. The premise is I can not use umount -l as I will have to destroy the partition after remounting the one, and I would like to avoid kexec due to not knowing if it will have the same buggy efect on this misconfigured VM of being unable to bring it up again. I am planning to have a compressed btrfs partition for /var/log and another btrfs or xfs for /var/lib/docker, and put all others together with the minimum required space as possible once they will be almost static. And in the future I may put them as squashfs together with the root one and mount a overlayfs to make it easy to detect misconfigurations. I would like to be able to do all of this without rebooting, though I don't know I will be able to.