Here’s a possibility based on your 200 miles a day, seven days door-to-door.
Pewsey to Harwich, to catch the overnight boat to Hollland is three hours. The boat does not leave until the evening, so you could maybe have a full day at home / work before departure. This might save you one of your seven days perhaps? The boat is an overnight sailing, so you arrive ready for a full day ahead.
From the Dutch port to the middle of the Harz mountains is roughly 340 miles. A chunk of this would (probably / maybe) be on motorways, so it would easily fit into your 200 miles a day target. Six (or at worse five, depending on how you treat the ride from Pewsey to Harwich) days left. Of these, due to the return boat back from Holland to Harwich also being overnight, you would effectively lose a day riding back to Pewsey from Harwich. Five days left, or at worse four.
You could spend four or five days doing non-touristy things in the Harz. Or you could look at some of the suggestions in the German section to stitch yourself together a different sort of non-tourist holiday by joining together several of the routes in the posts. You have a year to plan it, which is perfectly adequate.
Alternatively, you could have a look at the other ideas in the Benelux / French sections and start to stitch other ideas together and / or look at some if the trip reports. If those trips have taken two weeks, then adjust them or chuck in some motorway to save a lot of time.
In another post / thread you said you are familiar with RiDE magazine’s suggestions. Have a look at their website, as they have ideas for seven day jaunts, which might suit you, too. For example, picked up from another post and thread on UKGSer, you could do this and / or amend it to suit yourself:
In short, start to do some work for yourself, not least as you know yourself and those you go with much better than we ever will. Give it a go; then tell us what YOU have come up with. It’s fun and it’s free. For example, have a look at:
https://www.ukgser.com/forums/showthread.php/471784-Wallonia-aka-the-Belgian-Ardennes And then see what you can do by following links which take you off in other directions. For example to the ADAC website and maps, which are (with a bit of imagination) easy to stitch together. Or have a look at
https://www.ukgser.com/forums/showt...reat-roads-mate-GPS-files-got-a-route-mate-……
Some of the links will inevitably die as their websites move or are not maintained by their creators. But don’t despair. Use some imagination and look for the areas in Google and surf away. There are other posts on threads on how to do this. Here, to get you started is a link to the current ADAC page:
https://www.adac.de/der-adac/regionalclubs/nrw/motorradtouren-fuers-navi/
If you can’t make your own holiday out of this lot, you’d better stay at home
Tap around, you can’t break it. Here it is, translated into English, using nothing but an iPad