I then decompressed image.gz and tried to mount it by double clicking it (i.e. via DiskImageMounter.app) to make sure all the contents are there.
What you have is a full-disk backup. You haven't, but had you taken this while /dev/vda was rw-mounted, it would at least be slightly broken; it would be probably heavily broken if image.gz also resided on /dev/vda, as you'd be heavily modifying the file system during the backing up itself.
This means it's not only the partition that contains your data-containing file system, but starts with a partition table, and it seems it contains an UEFI boot partition.
Upon double-clicking, your DiskImageMounter seems to recognize at the very least that partition; that's good.
The others might or might not have been recognized; your operating system simply might not have file system drivers for that.
I'd say: if you want to access the data on that image, simply boot a VM using that disk locally.
If you want a more useful backup, I'd say you should simply back up the files, not the raw disk image (unless you really just want to replicate the whole VM). There's nothing outside of files and their attributes on a file system – so you don't lose anything just backing up the files.
And that could very trivially be achieved either directly using
ssh user@remote tar cpf - --xattrs --acls --zstd / > backup.tar.zst
which will just run tar on the target system to archive all files, compress it using zstd (needs to be installed on the remote system) on the fly, preserving all file attributes, xattrs and acls, and pipe the result through ssh to your local machine, which directly writes it to the local file backup.tar.zst.
You can also make a smaller archive of all files on the remote in a file on the remote itself (which excludes the backup itself), using mksquashfs (needs to be installed), for later download:
mksquashfs / backup-$(date --utc -Ihours).squashfs -wildcards -exclude '... backup-*.squashfs' -comp zstd
The elegant part here is that it's easy exclude any backup file to be included in the backup itself, and that mksquashfs inherently saves a lot of space by compressing well and stores the content of identical files only once.
Another elegant aspect is that these squashfs archives can directly be mounted (at least on Linux) – no decompression necessary. On MacOS, you can install squashfs-tools to uncompress, and I think (never tried), you can use macFUSE together with squashfuse to also mount them. Have fun!
/dev/vdapoints to this being a VM. The "easy" way here is just use the tools of the virtualizer to export an image.