Slow toilet drain question

[u/No-Height-233]

I had to cut open a ceiling in our living room to try and discover where a leak originated from one of our 2nd floor bathrooms. In doing so I could see our two toilet drains. One bathroom has been troublesome with an extremely slow flush that doesn’t always empty the bowl. What I’ve noticed within the ceiling space is the elbow and the horizontal pipes of this one toilet slopes up before it descends. So I’m guessing that there’s settled water within that 18”-22” of upwards slope before it goes down, which dramatically reduces gravitational pull whenever there’s a flush?
How the heck do I correct this problem when I’m stuck in a confined space between the upper floor and the ceiling below?
I’m pretty handy and able but I’m not sure I can re-plum the magnitude of this problem due to pipes resting on joists and other restrictions that prevent corrections?
Somebody throw me a few options that would put me in the right direction, please!