You do limit your choices a bit with the pen style. The T-whatever numbers do get good
reviews for those who use them, but it's a fixed wattage setup, which not everyone likes. The tanks are designed to suit the wattage range but coming from the ego AIO, they might be more than you want.
Pen style... Hmmm... Could buy a used provari and put your MTL topper on top, but they are expensive/hard to find, although probably the best MTL tube mod out there. I'm thinking... Honestly I usually recommend the Aspire zelos kit that comes with the Nautilus 2 tank. It's not a tube, but it has a very small form factor and an internal battery. Same with the pico kit, they offer both internal and external battery options, and many people like those setups.. I still recommend them even though mine was not my favorite setup of all time, but they do a lot right for the price point.
Best of luck,
Anna