BDSP was a terrible remake that was farmed out to a random porting studio. It's one of the lowest effort remakes I have ever experienced.
One of the biggest issues is BDSP added modern EXP share where all your pokemon gain EXP every battle.This caused a huge problem for BDSP because they didn't re-balance the game to account for you gaining much more XP than the original developers expected. The game is completely trivial because you become unreasonably over-leveled when playing through the game normally.
Platinum also added extra content to gen 4 beyond the original Diamond and Perl. None of this content was added to the remaster. If you are wanting to play gen IV and have hardware that can run it, Platinum is the best choice.
About SuMo and USUM, I played both as they came out and thought it was really fun seeing the differences in story, from the middle on it becomes like two entirely different games, if you are a fan I would recommend both. If I had to choose, I would say USUM is the more complete game, has more content and side content, more story and even an interesting post game story that is definitely worth playing for any fan of the older games, and I am legit surprised seeing people talk about SuMo being preferred because my experience with everyone I talked about this has always been people preferring USUM.
About BDSP, biggest complaint is how they remade the old games but did not do almost anything new to it, didn't even add the extra story from platinum which makes platinum be a more complete experience than even these remakes. They're fine but underwhelming.
So if you only want one, I'd say USUM. If you are considering all and deciding on the order, I'd say SuMo, then BDSP to take a little break, and then back to USUM.
The others already answered your question, but since you said you thought pokemon games were too easy and you have a steam deck in other comments, I would recommend Pokémon reborn. It is a free fan game on PC. I play it on my steam deck and it is great for it. Just takes a bit of setup and configuration.
That game is challenging if you want something difficult. You really need to plan out your approach as only brute force attack moves will get you nowhere unlike brute forcing a normal Pokémon game.
I never finished Ultra, but the consensus I've heard is that normal SuMo has the better story, Ultra has the better postgame content and is at least somewhat more challenging. Which one is better really depends on what you're looking for in a Pokémon game.
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
Well either way I’ll play on the steam deck, so not needing to switch between screens in a plus for brilliant diamonds.
In regards to s/m and us/um. What if I just want to beat the elite four which would you recommend? I’m not sure I care about the store or the post game content. I’m not even sure what post game content is, so maybe I’d care, but probably not.