When you leave Memmigen heading south, you are near the Austrian, Swiss & Italian borders, so all of these countries are less than a couple of hours gentle riding away.
Grossglochner is excellent, but awash with tourists and in many ways overrated. There are so many better passes in Austria that on one trip you will only see a few of them. The best, though not the highest is the Nochalmstrasse.
I suggest, if you wish to try the Stelvio, Leave Memmigen and head for Livigno. Make this your base for at least one day.
It is close to the pass. If you have not ridden uphill to tight hairpins before, do not cut the corner. Stay wide, on the wrong side of the road if need be. The inside line is often very steep, and if you are forced to stop by an oncoming vehicle, it often results in a "drop."
Enjoy your wanders through Switzerland, for you will be in and out of it round this area. Never speed in Switzerland. Fines are proportional to income. Swiss people are very efficient, but have had their sense of humour surgically extracted at birth.
In Austria, watch you speed in villages. There are cameras at many pedestrian crossing points.
Last but not least. Overlanders are Fabulous!