Oooh.... I've heard good things about the Kayfun mini, but I just can't justify the price. You know, I hated the original Nautilus, some people love the X, but it takes different coils, I can't believe the difference in vape taste from the Nautlius original to the Nautlus 2, and you can still use all the same coils. I can MTL on the 0.7 coil and actually prefer the taste... It's a super clean vape, but I enjoy the 1.8 coils also. I can't comment on the Kayfun but I'd LOVE to be able to, maybe at some point I'll buy a clone.
I don't exactly know what makes the Nautilus 2 so much better, but it truly is in almost all respects. I'd love it if the tank were slightly larger, but I understand why it isn't. It's a workhorse, however, and I only have to fill it back up a couple times a day, and I don't even get 2 mls in there due to the "fill to here" line. If you don't overfill and close off airlholes while in transit, mine doesn't leak. The first fill, I'll fill it to the very top with a new coil, as priming it will make the level go down quite a bit. It's a very flavorful tank, however. And the metal sleeve is a big bonus to me

(Clumsy).
I have the ego one mega, and that tank performs okay, but it's not my favorite vape, at this point. I bought it with those rebuildable coils to try TC, though, so once that happens I may feel differently
Best of luck finding what you need.
Anna