The classic filter is the external filter. Above a certain size, you can hardly avoid this type.
However, these are also available for small pools, the upper limit is ... let's call it enormous.
The main advantages are a large filter volume with a high water flow rate without an annoying block in the tank (pro-tip: glass inlet and outlet!).
But of course there is not enough space in the cabinet. And there is the at least theoretical danger of a leak. You have to decide for yourself.