I like slow trips, so this would suit me. But I am someone that prefers a week in Mittenwald to a week in Munich :)
On 2 trips to Salzburg, which has a compact core and is a lovely base, we have done day trips to Berchtesgaden for lake Koenigsee and Mt Jenner/ Untersberg/ Hallstatt/ there are more options.
Mittenwald is tiny, but you like walking and it has a surprising number of options for relaxed days out. We have had 6 nights there in 2 years and another 5 nights upcoming. Many people on various forums advise against it, but it totally depends on your travel style. Cablecar and walks on Karwendel, (a 5 minute walk from centre of town), the lakes (can get a free bus if you have your hotel card), walk to Leutaschklamm from town and do the boardwalk in the ravine, (so lovely, it's a flat walk), chairlift up Kranzberg then take one of the many trails, we intend to hire bikes (by train station) and ride to Krun next time, we trained (20mins)to Garmisch Partenkirchen for Zugspitze and Alpspitze last year, next year intend to do the Partnachklamm and trails there (handy beer gardens en route) and a Seefeld day trip maybe? BTW, the TI in Mittenwald is fantastic and has a great trail map. Lots of older Germans walking, few international tourists, but great signage everywhere.
Your choices are on good transport routes, so you have flexibility if you have days you'd like to fill.

Innsbruck is another great day trip from nearby Mittenwald. Or also a great place to spend a few days if small towns bore you after a few days - lovely excursions into the hills around Innsbruck - really sweet!

PalenQ
Many thanks for all the advice about trains. I really appreciate it and I'm sure they will be great
Our idea is to arrive at a place and stay in an apartment, unpack and go out from there each day into the surrounding area and explore, get to know the area, find a favourite bakery and cafe/ restaurant for the occasional meal. Thus I thought Bad Ischl would be central for getting to Hallstatt, St Wolfgang, salt mines and ice caves etc in thar area.
I thought from my research that there would be plenty to see and do including a bit of walking around lakes and down from mountains after going up in cable cars etc as well as perhaps the odd boat ride on lakes. Do you think st Wolfgang is central enough remembering that we will have a car to get around this area a bit? It does sound lovely. I get the feeling that you think we might not find enough to occupy ourselves ? ?

Adelaidean,
We sound like we are on the same wavelength here. We hate moving our base all the time so always look for an apartment as a home base where I can make the odd meal, wash clothes and spread out a bit and relax at night after a big day of exploring.
Keeping that in mind do you think we will struggle to occupy ourselves in Salzburg? It looked to me that there were quite a few trips to do from there including the ones you mentioned and being Aussies travelling for an hour or so to see something is something we are used to.
Mittenwald sounds great to us and we intend to visit Innsbruck from there as a day trip as well as other towns and mountain tops as you have mentioned. As long as it has a few cafes, and food shops ( like grocery stores for meat, veggies and other staples ) to suit our needs.
Do you think four weeks altogether is too long to spend in Austria keeping in mind that this is a holiday not a " tour" and we may not be coming back, there are other countries in Europe to explore next time.

While Salzburg is small, I still think there are lots of easy day trips which makes it a good base. I didn't even get to Werfen or the St Gilgen area. The simplest day trip, was bus to Berchtesgaden (40mins, bus leaves from convenient stop just near old town), I think we took an early 8am one, so caught the 9am ferry on the lake, so tranquil, just stunning. Be sure to stay on the boat to Salet (don't alight at St Bartholomew) and take the lakeside trail around the second lake (Obersee) to the beer garden, what a stunning place, nestled in a valley between mountains with turquoise water. The Hallstatt trip involves a few changes, a ferry and 2.5 hours, all well coordinated, and is a lovely worthwhile daytrip.
We stay in Haus Schweigart in Mittenwald, fabulous apartments. The local stores are a delight, tiny bakery, smallgoods store, and a fruit/ veg, but also a large supermarket (Rewe) with everything. We do love our food shopping there. There are nice restaurants too, a favourite Italian 'Da Momma Lucia" and an awesome rotisserie place that is hard to walk past (pork knuckle). The other great thing is every walking trail has a beer garden en route, often simple Bavarian fare.
I now lean towards regional trips. As you know, the long haul is a pain from Australia, I no longer want to add endless intercity trips. Our next trip is primarily north Italy (finishing off with another stay in Mittenwald), short journeys between destinations. It's a great way to travel.

Thankyou so much Adelaidean, this is just the sort of advice I was hoping for.

Yup all the day trips from Bad Ischl can be done from Salzburg but Bad Ischl or St Wolfgang IMO is much closer to Hallstatt - I'd suggest a few days in the Lake District doing day trips close to there and then much more time in Salzburg doing day trips from there to Germany and places closer to Salzburg - Zell am See is a very very pretty town on a placid lake that is one easy day trip and a scenic train ride to boot.

If doing lots of trains in Austria look into the Austrian Railpass which lets you hop on any train TML anytime - just show up.

thanks PalenQ I'm off to research Zell am See.

Zell am See - a wonderful town on a neat lake - we walked around the lake in a few hours but there are also boats - the Krimml Bahn scenic train leaves from here to to go into the hills:

https://www.google.com/search?q=zell...w=1745&bih=868

so many neat places so little time!

Zell could be a nice base for a few days and the Pinzauer Bahn (Krimml Bahn) is a neat scenic train from Zell to Krimml Waterfalls:

https://www.google.com/search?q=krim...w=1745&bih=868

PalenQ
Have looked at all your great suggestions and think that after some time in Munich and Vienna I will settle on a week in St Wolfgang so we can explore the lake area at our leisure and visit the likes of Hallstat etc then move on and have a week in Salzburg and go into Germany on day trips as well as the Zell am See area before moving on to Mittenwald for the final week.
Still planning, it's half the fun !!