Since Lumbermill machine might be on the pipe after watermill. I want to say few things.
Currently lack of people produce flour with a miling machine due its mechanics - if lumbermill will have same mechanics, there is few ideas to probably add some more effort:
1. Make new resource in addition to boards - Sawdust, it can be used as pawing instead of blocks of wood for champignons, also it should be possible to mulching it.
2. Ofc there should be some effort in amount of boards.
3. Would be fun if Sawdust from Giant Shroom Trees could be a spice.