I guess a bigger tank might be an option
It's quite common for fish keepers to want another tank (and then another, and another....) once they're up and running with their first - affectionately referred to as Multiple Tank Syndrome (MTS) on here - so you could could just view it as getting the second tank early! Although there'll be nothing wrong with your current tank provided it has suitable occupants.
It does look like your water is pretty hard - there's nothing wrong with this but it does mean you would need to look at fish which do better in hard water conditions. Ironically, the fish you have are actually suited to hard water, but aren't suited to your tank. The Endlers we've mentioned would therefore seem to be an ideal fish for your current tank if you choose to stick with it. If you haven't come across them before, they're quite similar to Guppies in certain respects but smaller
Alternatively, if you do go down the second tank route, I notice you mentioned you have an assassin snail and some shrimp - a 25l tank should make an good snail/shrimp tank.
As for your other water parameters; 0ppm ammonia and nitrite is good (these should be 0) and whilst 40ppm nitrates is a little on the high side it is still within acceptable limits.
I quite like to panoramic wall mounted tanks - i think they are about 50 - 60 litres perhaps this would be better.
Certainly the water capacity would be better for your current fish, although probably still not ideal for the Molly (
Seriously Fish recommends a 90cm x 30cm x 30cm tank for them). I haven't seen a wall mounted tank in the flesh before but you would probably need to consider the weight this would place on your wall, how easy it would be to maintain it once it's up there and also how wide/deep the tank is (front to back), e.g. even if the tank is very long, if it's only 10cm front to back then this would still limit your fish choices.