One week, one loop: Bled, Soča and Piran
Most visitors see Slovenia as a loop by car: Ljubljana, then Bled and Bohinj, over the Vršič pass into the Soča valley, and down to Piran on the coast. The country is small enough to do all of that in a week, and 5 GB for 30 days covers the navigation, the messages to guesthouses and the photos of a trip like that.
Ljubljana with a day at Bled fits in 3 GB for 15 days. Three weeks in a campervan, or remote work with a laptop on the hotspot, calls for 10 or 20 GB. The eSIM only starts counting when it first connects in Slovenia, so you can buy it as soon as the trip is booked, up to 180 days ahead, and install it whenever suits you.