I forget where the Nextcloud app stores all the files by default, but if they are within the webroot (i.e. /var/www/nextcloud) then using TKLBAM, something like below should work. Note that I split the tklbam restore into 2 separate steps. The beauty of that in this case is that all the backup files will be found in /tklbam-dump if you want/need to restore some additional stuff.

mkdir /tklbam-dump
tklbam-restore BACKUP_ID --raw-download=/tklbam-dump
tklbam-restore /tklbam-dump --skip-packages --limits="mysql:nextcloud /var/www/nextcloud"

Please note, I haven't tested that, it's just off the top of my head...

Also FWIW the current TurnKey appliances only have PHP7.0. As far as security is concerned, there is no issue there as Debian are providing support (despite it being EOL by upstream). Although I am also aware that as v7.0 is EOL, many vendors are now requiring newer minimum versions of PHP as a requirement. Installing a newer version of PHP is a bit of a pain as you won't get the auto security updates, but if you need a newer version, then I recommend that you check out https://deb.sury.org/. FWIW, he's one of the Debian Developers who maintains PHP packages within Debian.

That's also another option for you re your appliance; as TurnKey is based on Debian, you could alternatively do a "in place" Debian upgrade. I would highly recommend that you take a snapshot first so that you can roll back if it all goes pear-shaped, but it's an option.