Mandolin was my lockdown project too - meant as a bit of a joke, but thanks to a good instrument (Ashburys, £250 on eBay) and an excellent teacher, much to my surprise, I've fallen in love with it. The tuning in even 5ths makes a lot of the shapes more logical (as noted above) and a wider range easier to reach than on a bass. I struggled to pick out some simple Bach cello pieces on bass, but on mandolin they just fall into place under your hand (which makes perfect sense as the cello is also tuned in 5ths). The shapes are upside down and back to front (as also noted) but the thing I still find hardest to remember is that the you sometimes need to play notes on the 6th frets.
It's wonderfully versatile in genre, even just picked rather than strummed. There's the whole lute repertoire, some lovely Baroque stuff, Celtic dance tunes, folk songs, through to Grateful Dead, Rod Stewart, R.E.M. ...
It is technically much harder to play than bass. One of the first things my teacher said to me was "It's a difficult instrument to play", and the better I get the more I realise how true that is. The slightest variation in pick grip or wrist angle comes through in the sound. I could never have learned from videos - ymmv of course, but I need Patrick watching me and suggesting tiny subtle improvements.
And then I spotted an Irish bouzouki in Johnny Roadhouse - a relatively recent hybrid of a mandolin and Greek bouzouki, usually tuned GDAD. Filthy and battered, but still with a beautiful tone. I cleaned it, had a new nut and bridge made, and set it up in mandolin tuning with the G and D strings in octave tuning, A and E in unison - so the low notes have a rich resonance and the high notes are clear and pure. The wider fret spacing is easier than the mandolin, I agree, but the most important difference for me is that its tone range is perfect to sing with. A whole new world 🙂