Have been spending the last few days planning another trip. The bus that I have used in the past to ride to either Amtrak or O'Hare airport in Chicago has this luggage policy:
We allow up to 2 bags to be stored under the bus and 1 carry-on per adult fare
And their bike policy (I am going from Wisc to Illinois) is:
Note: Bicycles are not permitted with the exception of Wisconsin and Illinois routes and each bicycle will be charged $10.00 boxed, $20.00 unboxed.
From:
https://help.coachusa.com/portal/en/kb/articles/baggage
They apparently did not proof read their dimensions, but I suspect almost nobody reads that.

I probably will ride bike from home to the bus stop, thus happy to pay the extra to avoid boxing, especially if I use the Trainside/Carry On bike option on Amtrak. The taxi company in my community that had bike racks on each of their taxis went out of business, so getting a bike to the bus stop will be easiest if I ride it there. Hopefully it will not be raining that day, as I will have to put the stuff from my panniers into a duffle or two at the bus stop while waiting for the bus.

***

Flixbus for that trip does not offer a bike option, so not using that bus. Flixbus is more stringent at one large (airline size) bag and one carry on, with specific dimensions:
From:
https://www.flixbus.com/service/baggage