It is probably tight as can be. I had a similar issue on an older one and it took a lot of grunting to get it to move. This picture is a but confusing, is this sideways and is that the bottom of a sink or of a water tank?
You could always just cut the two pieces of PVC where it says “E84” then pull the whole assembly so you can work on it. Then simply put it back together with two shark fittings. No glue or anything necessary.
There is also the option to just replace all of that PVC with PEX while you are doing this. It’s a really cheap and easy material to work with and you do not have to worry about winterizing it. It also bends pretty easily around corners. The shark fittings if you use them just snap together and can be taken apart as well. Or you buy the cheap ones you just crimp on. Just a thought.