I just finished plumbing my 5K gal above ground pool and did a deep dive into PVC plumbing. So here are some of my thoughts:
Regarding the unions I would go with a name brand (Spears which I like a lot or LASCO) since those will come with EPDM or BUNA (different rubber materials) gaskets which will last you a long time. They are also going to be there in 5 years when you have to replace a broken part or want to add to your plumbing. They also screw way easier and are easier to glue than the box store alternatives. The reason why I like the Spears once specifically is that they are very compact. The the union screw part is only an inch long (see second picture) which reduces the strain on the liner for my pool.
Regarding ball valves an important thing to know is that a 1-1/2" ball valve is only nominal 1-1/2". The actual inside diameter is not the same as the pipe would be. So if you are looking to get the best flow for a 1-1/2" system you should use a 2" ball valve with 2" to 1-1/2" reducer bushings that will allow to connect your pipe. I think this is not strictly necessary but I wished I had known it before buying my parts. I would also just add one valve per side since they add a lot of bulk and are usually fairly expensive.
Regarding Flex PVC I used it for my whole plumbing (see picture) and I can only say that it was an absolute pain. From cutting it, glueing it to getting the right fit. It is way stiffer than I anticipated and I had a really hard time to make it work. Since it has a spiraling core it is really hard to cut straight and even just reaming it (beveling the inside and outside edges of cuts) is tricky sometimes. I can see how it has it's advantages for long runs but for an above ground pool with the equipment close together and space restrictions like yours I would not advice it. Besides that it is also more expensive than pipe and you also should use a special flexible PVC cement which also adds cost.
Hope that helps and is not too late
I plan to get rid of unions and threads altogether this season and go with camlocks. I have them, they are just sitting in my pool shed waiting for me to get around to swapping them in.
I got mine here - Polypropylene | Plastic | Poly Camlock Fittings
Good price, decent quality, fast shipping. Went the threaded ends, so I can glue on a threaded PVC adaptor and then thread on the camlock. Yeah, that does include threads, but I plan on never having to unthread the camlocks once they are on.
