A lot of medicines can be harmful to bettas.
He's probably getting old, male bettas are sold in shops older than females. Although the lifespan of a betta can be anywhere between 2-5 years, 2-3 is more common. I'm going through something similar with my elder female betta. I lowered the water...
Dwarf gouramis may or may not be aggressive so have a back up plan just in case. A honey gourami might work better but some people say they need to be kept in groups.
Albino corydoras get a bit big for a 50 l (15g) tank. You should try salt and pepper corydoras or another pygmy corydora species.
There are a bunch of plants that don't need nutrient rich soils that you can also try.
Anubias, anacharis, java fern etc.
My concern is that if you try the second method that you'll end up making the tank cloudy with dirt. And the first method might require recycling the tank.
Both are doable...
Can you describe the bugs a bit better? I can't quite tell from the pictures.
The "stretchy body with a shell" might be a baby pond or bladder snail, people see them as pests but they also do a good job on algae control.
Rummynose tetras are tight schoolers with a beautiful red face.
Cardinal tetras are very colorful
Guppies don't school but can be very colorful. Get males only to prevent breeding
You have a ton of choices.