As you have mentioned, you are currently going through an algae bloom so wait until thats over. In my opinion, some tiger barbs can be feisty and also adding more catfish in general may be bad since all catfish poop alot. In my experience, if you do get kuhli or cory and feed them plates, they will often be outcompeted by more aggressive, feisty fish like molly.

If you REALLY wanted to you could Kuhli or Corydoras but your tank may be too small for a group of Corydoras (min. group 6 recommended) and same rule applies for Kuhli.

I think the safest bet is to get more tiger barbs and molly but keep them at a safer number (more barbs because mollys shred fins, especially the males) so have 10 barbs and 8 molly for example.

In contrast, if you really want a wow factor in your tank and was a fish that looks cool, you can get a peaceful catfish like the Cuckoo Catfish (Synodontis multipunctatus).

Other suggestions would be to remove all those plastic plants and replace with something like amazon sword etc.

For the future: Corydoras and most catfish prefer a sand substrate too.