Trip to Croatia in September

Split is a long way from the the UK and I would suggest that 5 weeks is a bit tight unless you want to do a whistle stop tour. There is so much to see on route and Istria, Krk and the Lakes may soak up much of your time. Regarding your route. It really depends on which crossing you take but the  suggested routes above are well proven. Much of the north west German motorway system is undergoing a major refurbishment. It’s not easy to navigate around the roadworks but it’s worth timing your journey through Germany over a weekend if you can to avoid the commercial traffic. 

Whilst the route through Slovenia would not be my preferred route the section of the Italian A4 between Porto Gruaro and Trieste is still undergoing a programme of widening and I would strongly recommend you avoid this short section if you can hence Slovenia my be the better route.

Whilst in France this year we were pitched next to a very helpful couple from Germany and I was able to ask them for some route tips on crossing Germany en route to Croatia - to cut along story short their best advice to avoid the worst of Autobahn congestion (which they admitted is very bad at the moment) was to cross Germany on a Sunday when virtually all lorries are not allowed on the roads.