Baltic Germany stops

Wismar is a nice small city with an excellent hotel at the Markt, and a perfect base for a day trip to nearby Lübeck.
These two cities are in my opinion the most interesting and agreeable, and Rostock the least.

Stralsund: Isle of Rügen to the east of Stralsund is interesting with Jasmund National Park being a scenic overlook of the Ostsee. The Prora relics from the Nazi era are an "interesting" sight. Apparently the Russians stripped the Prora complex of anything of value including the plumbing. Many beachfront hotels on Rügen.

Also beach front hotels in the town of Prerow to the west of Stralsund.

The Prora buildings have now mostly been turned into luxury condos. They did leave a block or two completely as they were, with no windows and trees growing off the tops of the buildings, etc. Pretty interesting area.

Binz has beachfront hotels and restaurants. Nice beach, too.

"The Education of Rita" was filmed on Rügen island, which is a bit of a detour, but doable from Stralsund. A car helps though.
The beachfront hotels aren't in the cities you listed but can be found in the many seaside resorts, which are all a few miles away from these cities. The exception is Warnemünde, a suburb of Rostock, which is a pleasant resort with a nice mix of resort architecture and old captains houses. Quite busy though, since cruise ships dock there.
In general the good seaside resorts are to the east of Rostock, on the Darss peninsula, Rügen and Usedom island. If you travel by train Warnemünde is the best option, followed by Binz on Rügen island, which is 1h away from Stralsund.

From the cities you listed I like Stralsund the most. Lübeck is bigger and more important, but parts of the city were destroyed in WW2, and most churches, which were the model for almost all Gothic churches in the Baltics btw., are very empty today. Rostock is the least interesting, but the best base if you do day trips to other parts of the region.

We visited the Stralsund - Rügen - Darß peninsula area several years ago late September as a week long bike trip starting and ending in Stralsund. We rented the bikes in Stralsund.

We had heavy rain one morning in Rügen and shortened the day's ride to Sassnitz by taking a local bus. It looks like transit access to Rügen is possible from Stralsund. There is also a ferry between Stralsund and Hiddensee. The Isle of Hiddensee is a car free island between Rügen and Stralsund. http://www.germany.travel/en/leisure-and-recreation/germanys-islands/baltic-sea-islands/hiddensee-island.html

https://en.wikipedia.org/wiki/Hiddensee

Hiddensee [ˈhɪdənzeː] is a car-free island in the Baltic Sea, located
west of Germany's largest island, Rügen, on the German coast.

https://www.wikinger-reisen.de/radreisen/europa/7667R.php
Google translation German to English:

To the north of Stralsund is the Baltic Sea island of Rügen, divided
by lakes and bays. The chalk cliffs in Jasmund National Park are as
picturesque as if they had been created by their admirer Caspar David
Friedrich on his canvas. Not far away are the white beaches of former
sea resorts like Binz and Sellin, which shine in new splendor. In
combination with the [auto-free] island Hiddensee and the original
Boddenküste around the bird-like nature paradise of Fischland and Darß
this cycling tour shows you the German east seacoast at its most
beautiful side.

Because I wrote a lot, my follow-up is a Trip Report, here:
https://community.ricksteves.com/travel-forum/trip-reports/baltic-germany-fachwerke-landesgartenschau

My takeaway from this trip was that if you pick the right ones (for your own interests and standards), you only need to go to one Hanseatic/UNESCO WHS city and only one Fachwerkstraße town. (Opinion) I made the mistake of going to three or four of each.

I see you already mentioned it in your report south of Wismar, the castle of Schwerin is worth a visit. Had not enough time to walk around there, but think Schwerin is further a lovely place to consider too.