Caporegime
Classic problem, I went with an Eheim Pickup 45 in my 57L - adjustable flow rate from 50lph to 180lph. Another good option is the Elite Stingray 5. Just take out your existing filter media and float it loose in the tank, should be zero problems but you can always monitor ammonia/nitrite if concerned.
Aesthetics aside, you could go with an airline filter with such a small tank and only a beta, im not convinced they give good enough water circulation (and Beta's dont exactly stir up the water) but with the odd plant and a stir every so often i dont see why not. i'd keep the pump running for a week or so to allow the filter to seed prior to removing it mind.
i've used them in the past as an addition on a bigger tank hidden behind some decor and tbf it did seem to work pretty well.
I had a little play with the current baffle solution and I think I have it just right. It's agitating the surface enough but the flow isn't strong enough affect the rest of the tank. You can see in this video the plants aren't even moving from the flow so it's very localised. I put a bit of cut up filter media in the outlet flow nozzle