^ Guru didn't power up Vegeta, only Kuririn and Gohan. But overall I agree, I think it wasn't a regression in the sense he's evil again, because he "turning evil" like you said was more about wanting to settle the score --- the only thing he didn't let go of and still bothered him at the Cell arc, given his annoyance at Goku dying and then he never getting a chance (or so he thought) to fight Goku again... And then it culminates with the speech at the end, which sounded like the true progression that was missing, acknowledging Goku and letting go of his obsession with him. He even says "a Saiyan who loves to fight AND is kind... I should hate you... Good luck, Kakarrot, you're the champion", contrasting with the old Vegeta who should hate Goku and the one who now sees value in Goku's way of being, acknowledges why despite his immense effort he could never surpass him and makes peace with this. I think the Majin part was all a part of this development that was missing. Though what could have been repetitive I think would be enforcing the love he had for other ones with Piccolo saying "for the first time Vegeta is fighting for someone other than himself, giving his life for it"... Whereas Piccolo noticed how Vegeta stood for Trunks in the CGs and I think him offering Vegeta a hand there post the battle with Cell could be this subtle acknowledgement of this side of the not totally egotistical Vegeta showing up, which didn't need to be reinforced in the Boo saga, as it was apparent by then he didn't only fight for himself, though one could argue it was Toriyama's way of making that more clear.
Also, I wouldn't say Vegeta standing for Trunks in the CGs was a contradiction compared to how he acted not caring when Gero almost killed Bulma and baby Trunks... Just that Vegeta developed those feelings towards Trunks with the time they spent in the RoSaT... And that "siding with Cell" was way more his Saiyans side wanting a challenge, not picking Cell over Trunks, just wanting the challenge of the battle. I think the fact Cell reached completion greatly due to him and him seeing his son dead may have struck some deep responsibility upon him too. Future Bulma knew Vegeta had a soft side but she was surprised when Trunks told her that... Implying Cell arc Vegeta also grew in regards to his feelings.
I could grant though that the pragmatic Vegeta from the Freeza arc who generally sought safer routes, like wanting Kuririn to near kill him before Freeza turned into his true self, didn't show himself in the Cell arc, where he mostly got careless, too overconfident. It could be argued though that awakening a SSJ made him more that way --- Piccolo said he regained his pride upon achieving the SSJ, while in the Freeza arc he was always under an inferior position power wise compared to Freeza, not to mention Freeza likely resonating strongly to him too, as that wall that was tough to overcome, while his gains in the Cell arc, plus different perspectives of enemies who didn't have the same impact upon him, making him more careless in general.
In the Boo arc he also seemed initially careless, saying no matter how strong Boo was, let him awaken. But I think that may also be different since he was always talking from a superior position... from a power of SSJ2 perspective there, which he felt couldn't be overcame. It seems that Freeza arc aside, he always thought the Saiyans would prevail overall, with SSJ and then later with SSJ2. When fat Boo appeared half dead, he was ready to kill him due to the possibility of things going astray again and he creating another Boo, so it may be argued this perspective changes a lot when he feels the real threat of the opponents.