Hi @koyashi,
Welcome to the forum !
I’ll add to @Falcmike’s answer that the Z travel of the Z-plus is 100mm, but that travel is centered differently than on the HDZ, so that’s why you may have read about not being able to reach the baseboard with short endmills and/or thin wasteboard(s). As you can see in that post I linked, the ability to move up higher (that the previous stock Z-axis) was a design intent of the Z-plus. But in your case the real limit will be the Z clearance under the X rail anyway (especially considering you have to add a supplementary wasteboard on top of the baseboard).
The Shapeoko Pro has one extra inch of Z clearance (4"), so 120mm thick stock may work without any mods and if your usecase allows for not using a supplementary wasteboard (i.e. never cutting all the way through your stock…my guess would be that you don’t?)