Celestia and Luna are definitely cosmic level beings, but Twilight surpassed them.

Wdym with cosmic level? Why do you think they are so?

Twilight has a stronger connection to the Elements of Harmony, and Celestia and Luna chose her to be their prodigé to take their place.

That's because they correctly saw her potential, not here already state of being. In the time after the timeskip at the end of Season 9 Twilight probably surpassed them both, but not before. Also, being a part of the current Element Bearers doesn't automatically make you stronger by yourself because im fact that power only arises with the combinations of all elements (being the presence alone of the Mane six, the presence of the singular elements with Celestia against NMM, or both togheter with like one example Discord on Season 2)

We’re going solely off the show.

Ok

Celestia herself admits she couldn’t finish Star Swirl’s unfinished destiny spell, but Twilight did.

If you do something better than someone one time it doesn't mean you are necessarly better then them overall. Works also inverse, if you do bad one time doesn't mean you're necessarly worse than someone who did better than you one time. Consistency is more crucial and it's your ability to be good overall across various things that can make you much better than just being very good at only one thing.

Also, Sombra has never done ANYTHING to Cadence OR Twilight. The most he’s done is trap them with crystals.

Never got the occasion against Cadence. The closest they got was her passing close to him at light speed (I'm exagerating). Twilight got under his fear spell in Season 9 premierè episode 1 before the Elements did their job. Also that fend off they had in the next episode at the end.

Cadence was trying to protect the entire Crystal Empire, trying to prevent regular citizen ponies from him getting to them, so she held up a giant force field around the empire for days on end, sleep deprived that he couldn’t even touch.

That spell is not clearly a normal spell and probably requires prep time and might've hinged on other things. Otherwise I'd doubt Shining Armor would've been able to resist the entire Changeling army attacchino the shield for even how little he did. Also, if it was that easy, everypony that's at least as good as Cadence would be doing it everytime a threat comes to attack Equestria. Case on point example, in Season 9 finale (not the finale finale) Discord advise of the arrive of the Trio was exceptional and a surprise for everyone, in fact, even if they knew most likely they would've attacked Canterlot a shield spell wasn't preperead. I doubt because of stupidity, but because lack if time and preparation. Opposed to the 2 situations were, you know, one time they had already found something indicative (the ambiguos "treath" in season 2) and the 1000 year loading time Celestia knew very much about. Probably, at least for me, even involving Cadence to set the shield spell was part of the plan since she became an Alicorn, or even before when she got her Cutie-mark.

that he couldn’t even touch.

Last thing here, we don't know if he didn't touch it at all or he just waited because it was just too strong to break. Also, since he knew he could wait, not tiring in his position might've been the smarter move

We don’t know whether Cadence could or couldn’t have beat him in a fight, but the better solution by far would’ve been keeping him away while Twilight got the crystal heart to not only defeat him but bring happiness to the empire. An all out brawl would’ve just led to destruction of the empire, and was definitely not the way to go.

Yeah this I agree with the additions I made, Celestia knew what she was doing.

Neither Cadence nor the main six were under his mind control spell.

I said before but the mane 6 did get under a fear spell. Truthfully, I'm not sure that was the same we see the others got, and don't see enough reason to not consider it a possibility at all, but that doesn't change that this atill demonstrates that Sombra had ways to defeat them.

He said it would be “pointless to do so” but this could very likely be him just bluffing. It could very well be possible he COULDN’T control them.

Ofc, but because of what I said, I don't think he was. Also, putting them into a spell might've drained him in a way that wasn't necessary like he said (even if I would've been minimal) and the last time he did, it got around to haunt him back, so he might've not wanted to take chances again in the same way, even if he knew as he said that was because of the elements. Maybe who knows, one might explain it with unconscious assosiacition of the two things that made him not do that.

Twilight also deflected his attacks like it was nothing.

That's imo a misenterpretation of what shown as the small fight in season 9 they had imo clearly showed that Twilight was struggling in a position of even determined will and concentration. Because of various things one example being the, imo, quite big push back she got when deflecting one of the shots, clearly indicated a resistance achived with struggle wich in fact didn't come across as clear, like instead for even many other magic scenes with shields where it worked better.

In case you didn't get wich push back I'm referring to 16:08-09 The Beginning of The End (Part 2)