Near Lake Michigan Apr to May. Cool, very unlikely to freeze but that is possible. Some years can be wet. Should not be buggy, bugs come later. But I would carry a small bottle of repellant just in case.
I have not toured near the lake so I can't comment on routes. If I was doing that trip, I would probably bring a long sleeve jersey and a short sleeve. Some days I would expect to want long pants instead of shorts. The UP (Upper Penninsula of Michigan) would definitely be chillier.
I use this website to assess weather conditions in places I might go to. I picked Sheboygan as being about half way from top to bottom. You can pick other places on the map for more data.
https://weatherspark.com/y/14295/Ave...tes-Year-Round
Further inland, temperature will swing much more, as the lake buffers temperatures to some degree.