Ok folks - there is some development on this!
(for those who have NOT had the "use default browser" option due to using outlook for work via InTune / Company portal)
I had to reinstall Company Portal, and now it creates a separate profile on my phone for Personal and Work. For Outlook in particular, now there is a list of all my personal accounts, and a button for work, but instead of all emails for all accounts being listed there, the Company button takes me to another instance of Outlook where i only see work emails.
So now, on the personal instance of Outlook, the "use default browser" is there. That is the good news. Links for Linkedin now open in the app.
Links for Aliexpress (my go-to test subject for this issue, since it is one of the most annoying ones when they open inside Outlook) still open in Outlook. I'll message Outlook help to see what's up.