Build and Ex-Aid are my favourites, the latter of which I already have. Double is also high up there, his gimmick is also cool. And Decade just seems like a cool figure who could work in play and photos with literally any other rider, his ridiculous amount of cards don't take away either. There's also a couple of others with cool gimmicks, but no attachment to them (yet).

I would go for Decade then!

The thing is I'm worried that by the time I get to buying Build and W they might be out of stock or discontinued.
Build could also work well with Ex-Aid, because of HGF

I wouldn’t worry about that, as Decade’s release was accompanied by re-runs of ALL main riders so far! In general retail FRS riders can always be relied upon to come back in stock; that constant availability is one of the joys of model kits in general :)

You just about convinced me to buy Decade, good job, but now I found out that he's out of stock at Amiami so, no-go unless he's back by the time I balance my budget. I didn't know about the recent restock, but that's a pleasant surprise.
I wonder when they'll get to making kits of Reiwa Riders. If it is only after they've done all 20 Heisei riders, it'll probably be 2022 or 2023 at the earliest.

This is a thing I find sort of insane. For a line that started in earnest with Build, Zi-O (and Geiz , even!), and Ex-Aid... they still haven't done Zero-One? And no Saber announcement? I'd think that they'd at least want to do each year's main Rider, if nothing else... but nope! A ton of Phase 1 Heisei guys!

I also recommend Decade because he comes with belt parts (and in Build's case, an entire crotch) for the previous riders. So if you do him now, when you build the others, you'll have those belt parts ready to go and swap in during the build process for fun.

Whereas, I built them mostly on release and I'm just too lazy to go back and swap out the belts.

It will feel like you have a Decade figure transforming into any new FRS you get.

Hmm, Decade will definitely be on my FRS shortlist. I wonder what will they do for future riders. Include alternate beltstraps or wait and include them maybe with a possible Diend?

Yeah, it's... really weird to me? I'm sure they'd have very little trouble selling compared to the likes of Hibiki and Agito! And I sure as heck wouldn't mind picking them up...

When they announced Decade's Decadriver feature they also promised that every succeeding Rider would be compatible with it -- I'm not sure on the details, but it's either that they're compatible with the belt pieces included with Decade; or as you say, they might include new belt pieces when necessary. You won't have to worry about needing to buy another new figure for the sake of the gimmick - as long as you have Decade, you can put a Decadriver on whatever Rider you want. They've already advertised Ryuki wearing one, too!

I mean, the whole model kit line is currently under the tag line: "HEISEI RIDER PROJECT", so it pretty much lets you know that they're solely focused on heisei riders for now.