I have the same set up as you, except my pool is not semi-inground. Still, I try to avoid crawling around under my deck unless I have to.
When I hard piped my pool, I used some old sections of 6x6 timber I had laying around. I put a piece directly under my skimmer and under my return to support the pipes. I also put a few pieces under the pipe runs to the filter pad and use some 2 hole straps the hold the pipes down so nothing moves around. I do not have unions or valves near my skimmer. I do have unions next to my return, because that is where my inline SWCG goes - so I get to crawl under the deck 2x a year to install and remove it. I have unions and valves on both the suction and return next to my filter/pump. They do tend to take a beating from being in the sun and snow and general weather. I have replaced them (unions and valves) once in 10 years. I did use the standard PVC ball valves from the big box store. They get 'crunchy' pretty quickly. For me that is OK because I do not use them that often, and if I really have to, I can plug my skimmer and return. The Jandy valves are much better and when it comes time to replace I may spring for them. You can also rebuild the Jandy valves. I find with the unions, either the threaded rings crack (and I don't crank the heck out them either) or the O-Rings fail. O-Rings are easy, you can make your own. It is the threaded rings that are the issue. They are not a standard size, so you cannot just get a new one, and even if somehow you do, you would have to cut the pipe anyway to get it on. I'm really thinking of changing the unions out to camlocks in the spring. I don't know why I never had that thought before.