Can't get MongoDB started on 18.04

Ok so after several failures and online searching, I've come across this answer in StackOverflow, which basically suggests MongoDB 5.0 needs a newer cpu in order to run

I'll admit that I've not actually tried this yet (downgrading) due to the constraints (already wasted a full weekend on this) but it is highly possible to be the case given my cpu is quite old, and that when I downloaded the mongodb package (as in the Ubuntu one, not the 'offical' mongodb-org package, it worked. I believe this Ubuntu mongodb package is the 3.somthing version)

I'll leave this as a potential answer as I'm sure it may help someone with a laptop as old as mine.

Will edit if this works for me