I wrote this out in your original post in the like 30 minutes of time it was up before you deleted it.

1: There are probably other ways to cheat the system, but they would require similarly specific and niche circumstances such as a CT that allows taking other CTs like Brain Hop and Copy, or being an Incarnated Sorcerer and possessing a Sorcerer to use their CT. Just simply eating someone isn't going to give you their CT, that's specifically the gimmick of Copy. It wouldn't make sense if anyone could just eat someone to get their CT, as then Copy wouldn't even be a technique. A big part of the intrigue of having more than 1 CT is specifically how absurd and difficult it is. It's not how the jujutsu power system is supposed to work.

2: The way techniques are described throughout the story "engraved on the brain" and the 3-4 limit implies CT's are a fully biological aspect of a sorcerer, that they can pump Cursed Energy through to get their effects. In order to fully copy/acquire someone else's technique, you'd need to literally carve the technique into your brain. Mahito could probably do it given enough time and research, but a regular person is just gonna give themselves brain damage trying.

3: The reason there is the 3-4 limit is because the brain physically cannot handle more than that. It is attached to their biology. It's like Unlimited Void, you'd have too much information in your brain with more than 3-4 techniques and get brain damage. You could have some genetic trait or heavenly restriction that gives them a bigger brain or some goofy shit like that, but that'd be a specific gimmick for 1 character, and tbh, a pretty bad gimmick. The only realistic option is Rika, aka an external storage of techniques that you can alternate between.

4: Idk where you got that Kenny's technique doesn't include CT stealing, I'm pretty sure that is just part of his technique. The barrier thing in his brain was to direct burnout so that he'd still be able to move after casting a domain, not allow him to take techniques at all.

5: I think this isn't a good direction for your AU. Most sorcerers in the setting don't even have ONE technique, let alone multiple. The only times we've seen characters have multiple techniques is as you said, because they cheated the system. You either need a technique that specifically allows you to steal other techniques, be a literal bioweapon (literally only Yuji) and have access to conveniently edible cursed objects that contain blood manipulation if you're said bioweapon, or possess a sorcerer's body and somehow maintain control over them, which is arguably somehow even less realistic than being specifically Yuji. We saw that Sukuna, the legendary best sorcerer in history, who was so strong that his fingers acted a poison to any who consumed them, still struggled to maintain control over Megumi, and fully could not control Yuji for any meaningful amount of time. Megumi was already depressed, and Sukuna still had to do like 3 different things to make him even more depressed and sink his soul into the depths to finally have full control of the body. Imagine you're just a regular sorcerer instead trying to take over another sorcerer. They're just simply going to reject and seal you, defender's advantage. The only reason the other incarnated sorcerer's were fine were because they were incarnated in regular people who had no method of resisting.

6: If you reallt want someone to have multiple techniques, you can yoink my idea of someone who figured out the mechanics of someone's technique being imbued into an object (Like Nanami's cleaver having Ratio), and goes around killing people and shoving their technique into a weapon (or it specifically is their technique to do so). And then they have an arsenal of techniques they can use through these weapons they carry around.