It turns out there is an ancient undocumented config value that can be used…
occ config:app:set --value "/apps/files/extstoragemounts" core defaultpage
Beware: This is a global option[1]
Disclaimer: Since it’s not documented I can’t promise it will be permanently supported, but it’s been in place for >10 years through a number of code changes in that area. I’ll try to get it added to the docs.
[1] It doesn’t look like it would be overly challenging to make this a per-user option, but someone would still need to volunteer to implement the change. The code handling this is here.