I still have a Time Capsule with 2TB hard drive. Still going strong. Both the drive and the wifi. Kinda sad that they won't work for remote backup by end of next year. Gives me an excuse to upgrade to wifi 6 or 7.
1st gen AP Express is/was IMO piece of crap. It is a miracle that yours works at all today. They very commonly used to burn out early on and mine (the first model) was a goner after few years of use. And they were impossible to open without damage and Apple also glued the components so badly they were super difficult to diagnose and replace.
So, yours might also have some capacitors which are not up to par anymore and that alone might give you some problems not related to compatibility with the Extreme. But, I cannot be sure as I have not heard about issues with compatibility between Apple Airports - but they might exist (?).