can you post pictures of your kitchen and a floorplan. Base cabinets are usually 600 deep and wall cabinets 300 so your shelves would be a good fit at 300

I would talk to your pool builder about the pool detail. They are the experts.

all our spas with still water levels above shoulder height when sitting are the most comfortable to use for sitting and swimming and we found that allowing unsupervised kids to use our spas can be problem with damage and safety issues so water height should suit the sitting height of your adults

I have used the 58cmx 236cm, plus a corner unit Ikea Pax in my recent apartment reno (not built in cabinetry, as standalone units), and previously had the Flexi Wire/Wall solution in a rental property I was in within a built in. Given the situation you're in where you can't change the depth, I would go the Flexi Wire/Wall System. The 36cm is not deep enough to hang clothing and the other solutions (e.g. Elvari) are potentially okay, but I feel are more expensive (I haven't costed them out though from a configuration stand point though, to be fair)? I've also tried to get additional pieces for my Pax recently and it's so difficult at the moment... I'm pretty fussy with my wardrobes and I really liked how configurable the Flexi was. As an example, you could have different length rails at different heights for clothing (e.g. top rail 100cm for shirts to dresses, underneath it 50cm rail for shorts/skirts under shirts), so I feel you can fit more into it. You don't have that option with the Pax and are limited by set 50cm/75cm/100cm widths for segments of the wardrobe. I also didn't have any issues with the quality of the Flexi and I changed it around a lot, getting the right combo (and because I got bored). I'm about to do another apartment reno, and will be putting the Flexi range inside the built-ins there. Visually my concern if I hadn't used it before would be that the Flexi range looks a little cheaper than Pax, but once you have clothing and accessories in it you don't really notice. The wire shelves also don't get as dusty, which was a plus. It depends on what you're doing with the unit long term, but as a renter I also really liked how the Flexi system was configurable for my needs - whereas I wouldn't want a renter changing my Pax around!