I don't think it's as serious as that. Volvo explicitly mentions the possibility of ACC with a manual transmission in the manual for my car:
"To start the function requires the following:
• The driver's seatbelt must be buckled and the driver's door must be closed.
• There must be a vehicle (the "target vehicle") within reasonable distance in front of the car, or the current speed must be at least 15 km/h (9 mph).
• For cars with manual gearbox. Speed must be at the lowest 30 km/h (20 mph)."
I believe the car's system is fully designed and adapted for it. The only thing is, I really don't want to pay Volvo 50,000 CZK (~$2,200) when I can get the exact same feature for a tenth of the price.