What 20000mAh power bank to charge my iPhone and iPad at the same time?

For 4-5 hours on a train with an iPhone and an iPad, 20000mAh is tight, you can't mess around. Between Anker and Baseus, both are solid honestly, but Anker has a better reputation for long-term durability - seeing as you had trouble with your old one, that's not negligible. I'd tell you to make sure your model has at least two output ports (ideally USB-C + USB-A) and a minimum of 30W power to charge both at the same time without it being slow. Personally I'd go with an Anker 737 or something equivalent, it really performs well and you don't hear much bad about it after a few months of use.

Both are solid, Anker and Baseus are reliable. For a 4-5 hour train ride with an iPhone and iPad at the same time, you really need 20000mAh minimum yeah, knowing you'll lose a bit in conversion. Anker is a bit pricier but you know it's gonna last, I've had the same one for 3 years. Just check that it has two USB-C ports or USB-A + USB-C so you can charge both at the same time, otherwise you'll have to do it one after the other.

I was pretty bummed when my old one died too! For train trips I'd personally recommend Anker, I've had one for a year and a half and it still works great, the battery holds up well unlike some others I've tried. Baseus isn't bad either but Anker's really known for durability and they usually have better warranty!