I am one who just didn't like the pico. IDK why, it just didn't do much for me, I'm afraid.
I prefer Eleaf stuff and I do think they have the same parent company. Might want to look at Innoken for a more tube style setup and/or internal battery mod, they have some nice offerings. A lot of my single battery mods are tubes or no longer manufactured, so I'm not too much help there. If you think you might like a tiny internal battery mod that packs a punch you could look at the Tarot nano, I run mine about 12 watts and I'm fairly certain that you'd get a day of vaping out of it, mine recently took a swim in my large cup of coffee and emerged from the experience totally unscathed after a couple days drying it out. It's cute, it's small, it's reliable.
I also really think you can't go wrong with the Jac B sandstorm and it comes in two sizes I think, 23 mm and 25 mm so sometimes if using a larger tank, you can't go wrong. It's a DNA mod and pretty efficient with the right battery you shouldn't have trouble with more than one battery in a day.
There is also the Geekvape Agis which is a single battery mod that is pretty much indestructible and can be had at a fairly reasonable price especially during labor day sales, now that it's older two battery model the Legend was released.
Good luck deciding.
Anna