Re: Bottom of pool is very bumpy!
I think there's two issues that could/should be addressed - water flow under the pool and proper compaction of the base.
One thing you could consider for water flow and compaction is to construct a concrete footer around the perimeter of the pool. You'd need to probably dig down anywhere from 12"-24" and fill the trench with concrete and cinder blocks to create a wall around the pool base. The outside of the footer should have crushed fill stone #3 size (1/2" to 2") and, possible some drain pipes. The interior of the pool base could start with a layer of poured concrete onto which you place large stones (#2 or #1). If you don't want a solid base of concrete you could just do stones but you should use a plate compactor to create a solid base of stones that are "locked" together. Landscape drain pipes should also be used at this level with passage through the cinder block footer to relieve any water that might get under your pool. The final layers should be successively smaller stones or pea gravel with the very last layer being decomposed granite (DG) fill material. Sand certainly makes a finer/smoother base but I would be very wary to use it as sand is easily moved around and, as you have witnessed, you can lose sand and get an uneven base. DG is a bit more coarse but is also more dense and does not move as easily. Since you live in a cold climate where freezing and thawing is an issue, proper drainage and compaction is key to making sure frost heaves from freezing water does not ruin your pool base.
The only other option is to try to build a water-proof base using vermiculite board but that would require a much more detailed layout.
Just my opinion I did paver-stone hardscape in my backyard and artificial grass with a retaining wall enclosure and it took quite a lot of work to make sure the ground was stable underneath it all. Even with all the effort we put into it, there is still one small section of the turf where I can see a little trough that developed in the DG base layer because the gas line trench was not fully compacted after it was dug out and refilled.
16k gal IG gunite PebbleTec (Caribbean Blue), 18' x 36' free form with raised spa/spillway and separate rock waterfall. All Pentair Equipment pad - 3HP IntelliFlo VS / 1.5HP WhisperFlo, MasterTemp 400k BTU/hr heater, QuadDE-100 filter, IC40 SWCG, IntelliTouch/EasyTouch Controls
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