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.