We of course had a discussion on this subject. The usage of 大陸 and mainland was imposed on the Taiwanese sometime in the 50s. Prior to that people would use 中國, 支那, or at most 內地 by Chinese refugees.
A term that would make more linguistic sense would be 本土, which is the term used to describe similar island-mainland situation for other countires. Like you would never see mainland in a Hawaiian context translated to 美國大陸. 大陸 means continent, and when used in the context of continental, 美洲大陸 would instead be used. So it’s either 亞洲大陸, or 中國本土, or just 中國.