I just did that journey thus last summer. It’s far. I did pretty much nothing but drive and even then it was two whole days from the Croatian coast to Dover. Don’t drive unless there’s a really good reason. You can fly to Ljubljana, Trieste, Zagreb and Graz very cheaply

There may be border controls or checkpoints due to the refugee crisis. There are still poor bastards trekking through Slovenia en route to the north.

You can buy vignettes for Austria and Slovenia at loads of petrol stations.

Slovenia’s lovely but it can be surprisingly expensive.

tbh, I’d fly too, and rent a car. EZY STN/LGW-LJU is cheap, and they take bikes too.

Driving from Frankfurt to Ljubljana is ca. 8-9 hours, which is not so bad, but I’ve also thought it’d be convenient to leave after work on a Friday and stop over in Munich (about half way for me), only to find the Munich traffic and then A8 roadworks traffic, and then Tauern tunnel traffic meaning the short 4 hour drive on Saturday am turned in to 9 hours. If I do that again, I’m not doing an overnight stop until at least Salzburg.

Border crossing coming back into Germany – could be 0-10 minutes delay, or could eb a 4km queue depending on time of day and which crossing you intend to take. There are non-autobahn shortcuts that locals can advise. My delay last weekend was about 2 minutes for an Austrian police vignette check, 0 minutes autobahn border crossing, and coming back took the Fernpass which had no checks at all.