I have plumbed in two filters using side mounts and dual multiports and also on others gate valves to separate as you say for back washing. Top mounts could still be done. Takes a little outside the box thinking. The costs are quite a bit different to over here so I see your point but for the size of the pool and pump increasing the area of filtration will bring the water quality up as is already noted by valving the pump flow down.

18" gives an area of around 0.17m but a 24" is only 0.29m (sorry my spec is in meters) at $380 for a 24" and assuming original sold at 50% $115 the cost would be net $265 and you'd still have the smaller filter area. It does seem big pumps and small filters are the norm in the USA compared to Europe.