What's Frieza's next move?

He'll most likely rebuild his army and try to conquer the universe again, while probably avoiding earth in the process since he knows goku and vegeta can kick his ass ass with the power they've obtained. Also I don't think frieza knows that Beerus and shin are linked, so in his mind he wouldn't really gain anything by killing supreme kai (Though immortality is still an option since he might think it'll give him the capability to take on beerus).

Freeza gets over himself and becomes Beerus apprentice to become the next God of Destruction. He would literally have power over all of Universe 7. Beerus is basically Freeza who is sanctioned by the god of all gods and mortals. That asshole goes to whatever planet he wants extort whatever he wants with threat of annihilation and everyone in Universe 7 has to put up with it because Beerus is the strongest god and has power over everyone. Beerus isn't going to give a shit what Freeza does as long as he doesn't destroy all life in the universe like Zamasu's dumb ass. Freeza has more potential than Goku and Vegeta and can progress much faster. Freeza can easily be more appealing opponent to Beerus. Neither Goku nor Vegeta will ever take up the position. If Freeza plays the long game once he becomes god of destruction he'll gain greater longevity than his normal life span than can pull a Champa, gather the Super Dragon Balls for complete immortality then have no need to ever find a successor for himself. It puts him in a position where he can get closer to Zeno and find some weakness to him or close enough to manipulate Zeno into doing something stupid, like will Freeza 80% of his power and boom Freeza is the new God of Everything and rules over all.

Even if that doesn't pan out Freeza aiming to become god of destruction would allow him to rule all of Universe 7 and continue to do what he did before only now all the god just have to deal with it as long as Freeza does his job every once in a while, which is basically chose whatever planet he wants to destroy. Who is to say he couldn't just chose to destroy Earth on his first day and Whis just has to go with it because Freeza is his new boss?

Rebuild his empire/train to surpass Goku/Vegeta.

Why would Beerus care if Freeza conquered planets? He was cool with it before and if I’m not mistaken lower the mortal level would only happen if Freeza destroyed the remaining planets as oppose to conquering them.

Yeah, but Beerus wasn’t going to help Goku and Vegeta fight Freeza when he came back though. Whis ended up helping them, but Beerus seemed genuine about not getting involved in the conflict. Ideally I think a god of destruction doesn’t get involved unless the universe as a whole is threatened to be wiped out and if the Supreme Kai can’t handle it on their own. Freeza was never a wipe out all life in the universe kind of villain like Cell and Majin Buu. As long as he doesn’t wipe out the remaining 28 planets (which makes less sense now since he can’t sell those planets to anyone) he should be good with conquering them.

Freeza doesn’t have much choice though. He can only conquer 28 planets max. The other planets are lifeless now for some reason. His empire is going to be small. Unless he destroys 10 planets for no reason and lower the mortal level I don’t see Beerus caring much. Beerus isn’t exactly a benevolent deity like Kami was. He has always been amoral and lazy. I don’t think it’s really his job to stop villains unless they threaten the balance like Zamasu did. Also I don’t think Goku and the others would stop Freeza from conquering unless it directly impacts them or they are asked to get involved.

Beerus is so fickle though. He said he was supposed to kill Freeza in BoG, but tells Goku and company in RoF he doesn't care who wins and won't interfere in the fight instead of killing Freeza right there and then despite it contradicting what he said earlier. I don't think Beerus cares enough to actually kill Freeza or any villain unless he has to. That is why I find it unlikely he would kill Freeza or would follow through.

Well when I said benevolent I mean I think Beerus would actually have to care about the life Freeza would oppress if Freeza starts conquering, but not destroying planets, again. As long as Freeza doesn't destroy life I just think it would be in character for Beerus to not kill Freeza. If this was Cell, who just wants to kill entire civilizations because he enjoys killing people I could see Beerus killing him off for trying to get back into his old habits, because Cell unlike Freeza would wipe out life in the universe and lower the mortal level, which puts Beerus on the chopping block. Goku and his friends would help if their friends are threatened and said friends ask them for help, but I don't think it is a given they will help whenever Freeza does anything wrong and they don't know about it. Goku and Vegeta aren't proactive heroes, they are reactive. I just don''t think it is a given that Freeza has to be an anti-hero necessarily to live.

Have you people thought about the idea of Freeza expanding to other universes? That's the only possible option.

He already knows which universes to avoid (U6 and U11) and knows what the best warriors of the rest can do. He already can casually survive "Hakais" too.

That might be a risky prospect if he runs afoul of a God of Destruction less lazy than Beerus is (which is all of them). Even a "newborn" GoD, Toppo, could casually overwhelm Golden Frieza without putting too much effort into it. On top of that Frieza should be fairly (in)famous in the multiverse now given that he was one of the last ones standing in the Tournament of Power and helped ring out the mighty Jiren of all people. Every GoD knows who he is, and likely won't tolerate Frieza's shit.