Replace He Filter, Or Not?

Hiya!  I'm new to fish keeping (bar a small tank when I was younger!) And was looking for some advice on my filter. I currently have an aqua szut pat mini filter that came with my 54 litre tank. It's been difficult to look up information relating to that specific filter and I have finally found aquael also sell what  appears to be the exact same one, I think they call it ecoline though. I do know that it is adjustable, from 40-120 litres and only uses a sponge- no carbon or other filters.
The issue is that the filter is kinda driving me a bit potty! (I am willing to accept this may be hormones as I am due in February!) I can't seem to adjust the flow to a point where it breaks the surface enough for gas exchange but not so much that my fish can't swim in it! I have it 'aimed' at the back wall where the heater is but still have a current a bit strong for the fish as far as I can tell. On the lower settings it makes an aweful juddering noise and the flow is more like spurts. The setting I have it at now blows the plants a little on the other side but anything in the middle is caught in a storm. I don't think it's a great filter either. It has no casing so the fish keep nibbling at anything trying to grow in it and I use api aqua clear whenever I see particles in the water which is far more often then I would like and would rather not use it at all.
I am looking at the eheim pick up and would rather an over sized filter running at its lowest rather then a smaller at its highest but I admit I need to check dimensions!
My tank has been running for a month now, (sadly I did not know about fish less cycling otherwise I would have preferred to use that!) And the occupants are four guppies and three rescued white cloud mountain minnows  (they were kept in murky water with no water changes and two dead fish when I got them! ) So a bit higher stocking then is desirable, but couldn't leave the poor things where they were! I set the tank up with tetra safe start which worked great but a treatment for white spot seems to have killed it off again (or ammonia spikes for another reason?) So I thought I would 'start again' with a filter that might cope better! (And of course I would transfer the sponge into the new filter to save what I could!)
Any advise is very much appreciated!

Hi sue, yes that is the same one! Though the paperwork with the tank claimed 40-120. So not sure if maybe there was a translation issue or just different sizes.
I'll have a look into the aquaball as it's always good to get recommendations! I thought both pick up filters I was looking at had adjustments but I'll get to my local shops to have a proper look at one as that was info from amazon!
I'll see what fine filters I can find as I think my filter just has slightly larger 'holes' (mesh?) Then other filters I've looked at. Combine that with the fish keeping the outside of the sponge clean I can't help but think I have less bacteria space then I should have.
I have to admit I've not looked into the fluval range yet so ill see what I can find out.
Thank you for such a detailed reply as well! I really appreciate it. And I'll let you know how I get on!