I’ve played both Diamond and Brilliant Diamond. To be honest, I don’t get where the hate is coming from for BD. It’s got a ton of quality-of-life changes compared to Diamond. If you had the choice between the two, I would certainly recommend BD, especially if you’re used to the the QoL from the newer Pokemon games. Whether or not BD is worth the money is a separate question entirely… but at least that can be solved by consulting some online “vendors,” if you catch my drift
As for S/M and US/UM, what I’ve heard is that S/M has a more coherent story. US/UM apparently chose to abandon the story near the end and shoehorn in an incoherent ending, with the intention of using the new ending to build up a stronger post-story game. The choice boils down to whether you care about the story more or whether you care about the post-game content more