ursamajor-rEFInd VS alma

Have a look here for some ideas.

I have never tried efistub to be able to provide any insight there, but since you say you like pretty things, GRUB is also themeable, as is rEFInd. I switched to rEFInd a while back solely because (again, though the default is sinfully ugly) it has great themes available. I ended up settling on the Ursa Major theme. I offer this only as a suggestion for possibilities to look into, particularly if you ever want to consider dual booting.

Check out ALMA for creating a bootable USB with preset files for custom packages.

I'm a big fan of this project for managing portable arch installs: https://github.com/r-darwish/alma

This above wiki led me to ALMA https://github.com/r-darwish/alma

Something like this would work to setup a portable USB install: https://github.com/r-darwish/alma. You can install whichever DE you want on top of that.

I'd recommend Arch Linux for developers.

It takes a little bit of set-up / learning at the start. But you can easily try it out with a tool like ALMA - https://github.com/r-darwish/alma

See my preset files here - https://github.com/jamesmcm/arch-i3-usb

But PopOS is probably the closest to the previous beginner Ubuntu experience now.

As a bonus for the only thing I can possibly think your trying to do, here is ALMA, the lazy man's Arch Linux Mobile Appliance(usb) creator.

I've used alma to make portable Arch installs for a couple of years with good results. Maybe give that a try if you can't sort the issues with your current portable install.

You can use ALMA to create a persistent USB disk - https://github.com/r-darwish/alma

I'd recommend checking out ALMA for creating pre-configured, persistent USB installations. Here is my collection of preset files.

Although I like the project and look forward to testing it (and possibly switching over), I really don't like that Alma already is the name of a linux distro (https://github.com/r-darwish/alma).