I was in the Harz last week. It was an organised trip by Railway Touring Company, so didn't have to worry about my own travel and tickets as we were provided with 3-day rovers and accommodation in Wernigarode.
From my experience:
The Brocken was worth doing for the experience, but definitely worth getting an early or later train up to avoid the crowds. It's quite an experience sitting in the siding on the way down watching another steam train thrashing its way up past you at speed.
There's some interesting bits and pieces at the top as well.
There were a lot of steam loco substitutions with diesels. At least one diagram was substituted every day we were there, and two on some days.
At one point, the entire line between Nordhausen and Eisfelder Talmühle was completely shut for an entire day, with no explanation other than 'Operational Reasons'.
Also on several days, the Brocken line was closed due to high winds with services terminating at Schierke.
The timetable changes are listed daily at
https://www.hsb-wr.de/Aktuelles/Aktuelle-Fahrplanaenderungen/ and it's worth checking last thing at night if you plan an early start the next day.
If you want to do steam in/out of Quedlinburg, you need to start or finish at that end as there's no steam diagram the other way. We ended up taking the bus from Wernigerode in the morning, having a look round the town, then doing the loop back via Eisfelder Talmühle and Drei Annen Hohne (including the balloon loop at Stiege).
It's definitely worth doing, especially as the Selketalbahn section is going to be closed from next year for 5 years.
Also, at 1414 there is (theoretically) a parallel steam loco departure from Eisfelder Talmühle, but all the time we were there, the train to Drei Annen Hohne was a Harzkamel diesel instead.
The same issue applies if you want to do steam into/out of Nordhausen, steam only runs out of there in the morning, back in the evening, with only railcars in the opposite directions. There aren't any suitable rail connections to get there in time if you want to start from further north than Ifeld.
It did look like it was possible to get the bus from Wernigerode to Nordhausen early in the morning in time to catch the steam train back north at 10:33.
However, the connection between the two buses in Benneckenstein (265 to 23) is currently broken due to road construction work in the town, and the latter truncated to a temporary stop to the south.
We never managed to do the Stiege-Hasselfelde section.
Hope this is useful.