Does all data fit on the 10TB? That would make conversion less painful,
Synology can only expand with the bigger disks, so you need to start with the 6 or 10TB.
Both pools need conversion and I am not aware of a supported way to do this without "redoing" all packages. You will have to HB them and restore on the new volume (this is not too difficult anyway).
You can export all data to USB and restore, and HB the packages to USB as well, that is probably what most people would do.
I think my path would be something like this, as internal move is so much faster than USB export/import:
The process is basically meant to first have all the data on one 14TB disk, and then create an SHR1 from the existing disks.
0) make sure your backups are done. This process moves data and there is no redundancy during the process.
1) pull the two 6TB disks from a properly shutdown system.
2) Install one 14TB in a basic volume, BTRFS
3) Move (using shared folder location change!) all your shared folders from the 10TB to the single 14TB
4) HB the packages to the 14TB as well, you may also export system settings to be on the safer side.
5) shutdown
6) Pull the two 10TB
7) Startup and make sure the NAS works and contains all the data and HB backups
8) shutdown and add again the 2 6TB disks as a separate pool, make sure these are imported as existing disks!
9) Move (using shared folder location change!) also all your shared folders from the 6TB to the 14TB
You should now have a working NAS with all your data on one single 14TB disk
10) Now you can setup the final SHR1:
e.g. you want the SHR to be 10+10+14+14:
Start adding 10+10+14, initialize it as new pool with one volume SHR1 BTRFS.
11) Then move back all the shared folders to the newly created SHR1, and restore the HB with packages.
12) Finally add the last 14TB that is now empty to the SHR to get 10+10+14+14
A lot of work, with plenty of opportunities to lose your data so take care!