Well, better late than never.
At some point Ubuntu LTS 20.04 was released, but exactly at the point honestly I was quite busy to build usual mongodb packages for it. And then later found out that 19.04 packages work just fine for it. So I used them, but always kept in mind that maybe it would be nice idea to build normal, 20.04 native (with all dependancies proper) packages.
So I did (after year and a half, haha, half a year before next LTS). Here they are, mongodb armhf packages for Ubuntu 20.04
There are high chances that 22.04 LTS packages are going to be released way sooner, because now I have my build machine ready and skill is pretty fresh. Unless the new Ubuntu will make it even more complicated than it is right now…