I am looking for a reverse osmosis countertop unit so that I can install myself without needing a plumber.
Any recommendations?

https://a.co/d/gXbjwrt

Had this one for a couple years before moving to a different home. Easy to install, easy to maintain. As long as you can drill a hole in the countertop yourself you can handle this.

It also had one of the best pure to waste ratios at 1:1.

I don't know exactly what a countertop RO system is given that RO needs a pressurized supply line to feed it (and an unpressurized drain line).

But an under-the counter system does not need a plumber if you have minor DIY skills. Something like this (although if you want to use the countertop spigot you may need to cut a hole in your countertop): https://airwaterice.com/collections/drinking-water-system.

Generally with RO you want at least sediment and carbon pre-stage filters. 2 carbon even better if you have chloramines in your water - otherwise you run the risk of early failure of your RO membrane which is the most expensive part to replace.

Just tagging this thread for later.

Look at WatwrDrop. We have a tankless I it that has been amazing. Super easy install, and takes very little room under the sink.