Travelling from Nagasaki to Sasebo with large suitcase

I will be departing Nagasaki on a Sunday morning to catch a ferry from Sasebo at 12pm. Initially I had looked and seemed as though a willer bus would be the best way to go, however now I am unsure about if a bus can accommodate me as an international traveller with a large suitcase + backpack, it seems there is a 10kg suitcase limit whereas I may be closer to 20kg.

What would be your recommendation on the best transport to use to do this trip? Something with a guaranteed/booked seat would alleviate some worry about missing the booked ferry too, but most importantly is that I won't get turned away when I arrive for having a big suitcase with me.

Thank you 🙏