Basing near Berchtesgaden?

I wouldn’t take a night away from Berchtesgaden to stay near Neuschwanstein an extra night.
Garmisch is much more appealing for that extra night.

Or just another day basing out of Berchtesgaden?

We based in Hallein for 5 nights in 2018. It’s about 45 minutes to Berchtesgaden, 20 minutes to Salzburg, an hour to Hallstatt.

All of those areas are gorgeous, as is the area around Werfen, just south of Salzburg. I recommend the fortress and ice caves near Werfen. Also, tandem paragliding over Werfen or Salzburg. What a thrill!

I'd go straight to the Berchtesgaden area from Munich, but if the castles are a must for you then stay the rest of the day and night in Füssen – it's a smaller but prettier town than Garmisch-Partenkirchen. There are many options when making your way from Füssen to the Berchtesgaden, depending on how early you leave. The quickest way will take you close to Benediktbeuern Abbey (if that interests you) and the small, attractive town of Bad Tölz.

It's easy to spend a week in the Berchtesgaden area without travelling too far for day trips. We cancelled our 2020 trip and were going to stay in Ruhpolding, just up the road from Berchtesgaden. I'd also consider Bad Reichenall and Ramsau as bases.

Berchtesgadener Land is one of my favorite places I have visited, but I'm an outdoors kind of guy. I stayed at Ramsau bei Berchtesgaden. Gorgeous. David Harper, one of the owners of Eagle's Nest Historical Tours, has written an excellent guide book for Berchtesgaden, which is available at Amazon: https://www.amazon.com/Your-Complete-Guide-Berchtesgaden-Obersalzberg/dp/3944501764/

I wouldn't skip Neuschwanstein. It's a beautiful edifice. As long as you appreciate it for what it is - the fantasy of an eccentric king and a tribute to Wagner - not a medieval castle, it's a worthwhile visit. But one night is probably sufficient.

The best way from Füssen to Berchtesgaden is probably by rail, back to Munich, then to Freilassing and from there to Berchtesgaden. If you time it right, spend an hour or so changing in the Munich Hbf and have lunch in the food court just off the end of the tracks in the main shed.

If you do the boat trip on Königssee, be sure to go all the way up to the end of the lake, to Salat, and hike over the bar to the Obersee. Anyway, it's easier to get on the boat at Salat and go all the way back to the start at Schönau then it is to fight the crowd to go back from St Barts.

I agree with Gundersen, don't bother with GaP. I've stopped there two times (plus changes a few other times), but I wouldn't go out of my way to stop there again.

We’ll have a car after Munich so we won’t have to muck about w train connections.

So based on this advice (thanks everyone), I’m thinking…

we leave Munich, visit Hohenschwangau or Linderhof (?) in the afternoon, sleep füssen, visit Neuschwanstein as early as possible the next day, maybe visit the third castle left from the previous day and head to Berchtesgaden w a possible stop along the way as described above.

..In that area 5 nights maybe staying in Ramsau with day trips and outdoor stuff (including paragliding!) in that area.

If you're in the area I would definitely visit both the Neuschwanstein and Hohenschwangau castles. They're both fabulous in their own way and offer a great contrast between them. Plus the little village of Hohenschwangau is fun to explore.