See the airport section here
https://en.wikivoyage.org/wiki/Ljubljana
If you are happy to take the bus out there is no reason to take a taxi back.
The 1810 trip should be fine for your 2020 departure, unless you foresee complications with checking in (such as visa issues, excess luggage etc). However a rare unexpected event could happen which may make your flight unsalvageable. You may have had enough of wandering around earlier on, and want to chill at the airport anyway.
Thanks for your input.
I don’t mind taking the bus to town since I don’t have “anything” to catch. The bus also allows me to do a little bit of sightseeing as I have never been to Slovenia.
Going back though, at least for me, is a different story. I will have a flight to catch. I assume a taxi will get me to the airport faster than a bus. Also from what I have researched there’s really not much at LJU; a taxi will allow me to spend more time in town.