First off, as others have stated, the OP does not use "jumping the shark" correctly.
Second, I disagree that the show was OK/good/great and then suddenly was not. It has been much more of a roller coaster for me. Sometimes excellent. Sometimes not. Sometimes just outright dumb. Sometimes all within a single episode.
I have watched & rewatched and am in the process of rewatching again before S5. Generally speaking, I can say the show really only lost me twice: end of S2 plot stupidity and the cause of the Burn in S3.
Don't get me wrong, there have been frustrations. Silly things in episodes I otherwise liked. The hokey-ness of the light years away Burnham/Sarek mind-meld in Binary & Lethe. Landry stupidly letting the tardigrade out in Butcher's Knife.
Sometimes, The cosmic level stupidity was too much. No one caring about Nahn possibly suffocating to death whilst Burnham has angst about offing Ariam in Daedalus. Or the explanation for the Burn.
Sometimes, the show has someone explain away the stupidity. Burnham justifying the galactically stupid decision to not simply jump away at the end if Valley of Shadows. Or Georgiou telling us how smart the plan was after the improbable 2 v 20 battle on the Charon succeeds after 1 shot from tiny Discovery rattles the whole city-sized ship (except Burnham & Georgiou, of course). Yes, good plan! It was absurd.
I mean, if huge plot stupidity is a problem for you, then the end of the MU, S2, and the Burn explanation are a problem. The S1 finale does not make much sense either. To me at least.
But outside of that, and the mostly terrible fight scenes, and the sometimes smaller stupidities within an episode, it has been pretty good.
I thought the beginning of S1 was excellent. And even with the dip at the end, I think S1 DISCO makes a case for best S1 outside of TOS up to that point (better than TAS, TNG, DS9, VOY, ENT). And a couple of the MU eps (Despite Yourself, Wolf Inside) were pretty good, as was the penultimate, War Within, War Without.
The first part of S2 was intruiging with New Eden and If Memory Serves the clear standouts for me. Heck, Perpetual Infinity did not seem as bad the 2nd time.
I thought the beginning of S3 went a long way in climbing out of the rubble of S2's ending. Hope Is You Pt I, People of Earth, Forget Me Not, & Die Trying were an very good start in my book. Kweijan was cool. Ossyria even had some cool potential. The Die Hard stuff at the end was cool. The Burn explanation and Vance making an impossible proposal to Ossyria were real dumb, though.
S4, they struggled a bit more early, but stuck the landing. Finally.
Ups and downs for me. All the way through.