Just my two cents, but you can find horror stories about just about any device that's likely to fit those parameters. I know the iStick 50Ws have been particularly bad on this front but my point is that with enough searching, you could probably find a story about almost any of these non-removable battery types developing a serious issue. I like the ones with built-in batteries, too, but without the ability to take them out and the fact that many popular APVs are made in a country with different quality control standards, some problems are bound to arise.
I've been vaping for a really long time but only recently made a switch to VW instead of a mech and a VV device, so I'm probably not the best to give you advice on what to get but I still thought the above was worth mentioning. I wouldn't pretend to know what the numbers look like, but there have been a LOT of iStick 50Ws sold and most of those customers remain happy. That said, I have read about and understand the problems specific to the 50W and I don't blame you for feeling nervous and wanting to make a switch.
For what it's worth, I have a SmokTech Xpro M80 Plus and an iStick 30W, both of which have given me no problems yet and perform great for the price (I'm pretty broke). But to my point, I've definitely read some rough stories about the Xpro-- especially the early models-- so I just know I've got to keep an eye on it. While there's really no such thing as too much knowledge and/or information and safety should absolutely be your number one priority, it's also be easy to make yourself crazy sometimes.
That all said, again I totally get why you want to switch. I'm sure someone else will know where to point you.