Saanim

A simple solution: To harvest rainwater in separate reservoirs where the rainwater will sediment (1/2 - 1 day, a regular addition of alum - SaanSom , so easily available and cheap in Thailand can improve the water quality). Then with the intake few cm above the muddy bottom you get quite a clean water for addition to the other general water storage, hence, reducing the water hardness substantially.

Your sand can serve you surely for years once you regularly backwash it. A quartz sand is laying in river and creeks for millions of years, always clean. (But I do not sell the river sand.)

On your question of pump replacement: you can take any pump 1.1 - 1.5 kW as you find on Lazada or Global House for some 8,000 Baht. It will serve you well over years as it serves me. On your question of piping: you should draw a diagram how the pipes go, to understand it better. In general, all sucking pipes (at the pool bottom) have to go into sucking side of the pump, the outlet from pump goes to filter. And from there to all inlets into the pool. Important: each inlet and outlet pipe should be controlled by a valve in order to regulate an equal sucking intake and an equal feeding input flow. (Or any flow in different pool location as per your liking).