Never had to deal with this myself but this link seems to have a very considered approach to treatment. https://www.pondplace.com/resources/blog/controlling-string-algae_ae8.html#:~:text=String%20algae%20are%20fibrous%20green,water%20and%20chokes%20the%20pond.

Your pond is small enough that it seems reasonable to me the algae could be removed manually.

Has anybody here has had an actual experience with these products? Have tried a few but they don't seem to work. I see plenty of clean small ponds, wondering how other people keep their ponds clean.

You can buy a Plecostomus for your pond at a pet store. They aren't winter hardy, in most places but love algae and grow quickly. I'm not a fan of additives for my pond.

I have the string algae issue too, and I use a net to scoop out as much as I can (sheets of it stick to the net), knowing I'll never get it all since I cannot reach as far as the back of the pond. I also have a barley bale in the pond (held with a string so I can haul it out later when it completely sinks). I did have a plecostamus years ago, and as I recall he really cleaned up the pond and grew huge. Cannot remember if he made it through the California winter or not.