I bought a fretless relatively early on, unlined, and found it surprisingly easy to play, I think being a finger-per-fret player makes it easier, as once you've found a position you aren't moving you hand up and down.
I've tried lined and it makes you obsess about being in the right spot, without lines you relax (which improves things) and rely on your ears to tell you if you are out (which improves things even more). The other day I tried playing scales along each string, eyes closed. I made any tweaks necessary as I went but was pleasantly relieved to find myself finishing in tune with the first harmonic.
I like side dots as if you make a big movement, you do need some landmark to make sure you start off close to in tune! In between ones are a nightmare.
After a practice with the new soul outfit last week I got a text to say "Fretless bass was an inspiration. Thank you" which helps make it worthwhile 🙂