How to get volume back to read-write mode

tijgert , to add space, you'll need at least two larger drives.  But now that the damage is done to the volume, more space won't fix the problem.  It will never put itself back in read/write mode and when I tried to force it via SSH (which you've already said you have no familiarity with), nothing "took".  Since you have a current backup, you just need to bite the bullet and destroy the volume, re-create it, and restore files from your backup.  Every time you re-boot, you risk losing the volume completely, and thus having to delete it or even do a full factory default.

If you are going to add space, the time to do it is after you delete the volume and before you re-create it.  That way, there will be only one re-sync except the short ones for the OS and swap partitions that occur when you swap a drive.  I would certainly go with something bigger than 18TB.  Assuming you do swap out only two, the new RAID group will be RAID1, so you will only see half of the additional space.  Two 18TB drives are a big expense for a 2TB increase in size.

A couple notes for you both:  If you are saving and restoring a configuration to get the shares and permissions the same, the new volume must have the same name as the old one.  If the volume is the primary (or only) one, it will contain home folders.  If you have two volumes, it will create new home folders on the other one and designate it as primary when you destroy the other one.  But, it doesn't move the contents.  If it is your only volume, then restoring a configuration will not create home folders   That's normally done when the user first logs in.  But there is a program you can call from SSH that will create them (one at a time).  Execute mkhomedir_helper <username> for every user (including admin) to create home shares.  Make sure you do that before you try to restore any files to home folders.  Otherwise, the restoration process will likely create a normal folder instead of a BTRFS sub-volume for the users, which will be problematic.