I was not not able to get Genos packs on the Yamaha site to load into the SX900.
I too was looking for more orchestral packs and did not find much. Website sites of people offering them are scattered and some look amateurish and frankly not inviting to lay down a credit card. There does need to be a comprehensive list of 3rd party expansion sources that offer quality SX products.
I am not that impressed with all those free packs Yamaha offers. They seem quite narrow in focus with instruments and styles from Eastern Europe, Middle East, Africa and Asia, unless one is looking for those specifically. If you want World styles these might be for you. The Christmas & Church pack and the Russian pack are OK, though I feel the sound quality of the voices are just average and inferior to the S.A. presets on the SX900. The Styles from these two packs are good.
Though I have not tried their packs, I think Dynamix-Audio's website looks great
https://dynamix-audio.com. they have a nice selection, I would try them. Second, I have heard good things about CMS Sound Design
https://www.cms-sounddesign.com and they also have a freeware section and made a nice effort to have English translations on the site, as the company is located in Germany I believe.
Another company I would look at is EasySounds.
https://easysoundsshop.de/en/yamaha-electronic-keyboards/yamaha-psr-sx900700/I did buy their "Dreamscapes" pack for about $73 and would recommend it for its cool atmospheric/new age/chill/ambient beat kind of styles, voices, registrations, & multi-pads. They say half of the styles in this pack are based off of a few of the existing styles in the SX900, most coming from page 9 of the Dance styles category I suspect. I was expecting a bit more usable voices from this pack, as most seem to be sound effect type pads and ambient effects, which work better as part of the styles than for solo playing.
You mentioned Soundwonderland. I have not tried them yet. I just wish they would be more clear in what they are selling. They need to re-design their website. I want quality voices and styles, and I want to hear what they sound like. I am not interested in sounds or styles inspired by certain songs, if that's what they are selling. Perhaps if they see this thread they can help explain, because I am interested if they have some good stuff.
My suspicion is that there are not going to be a lot of great expansion packs for the SX line, as Yamaha does offer a lot of voices and styles in the SX presets and they did make effort to make most of them sound really good. As for orchestral sounds, I don't see much but again Dynamix and CMS has some selection. Same was true when I had my MODX (returned it). However MODX/Montage have the free Soundmondo resource where you can download other people's styles and voice patches all for free. PSR does not have this type of resource.
Regarding best way to manage 1GB expansion, I think best bet is to combine your favorites using Yamaha Expansion Manager on your computer. It appears you cannot add packs directly to the keyboard without wiping out the other expansions already in there, so this seems to be the only way. I am also hearing it may be possible to play styles off of the USB stick directly. I am a newbie with this instrument so someone else may have better advice.
Let us know if you come across any other good packs.