When I first handled the 125 and 150 in the store years ago, the 125 felt more comfortable. After buying and using the ro125, I never really liked it and always used something else. Not long ago, I switched to the ro 150 and it is like day and night. I don’t know if something was wrong with the 125 but the 150s extra size and weight seem to really calm it down with the way that I use it
You want to first cut it, either manually or with a router.
Then for the sanding step the Rotex would work well and as mentioned 150 is the preferred size. But you still need to flush-cut or almost-flush-cut the material before sanding.
Veritas makes a really good flush cutting saw. More geared towards cutting off plugs, but might work for you. One size of the blade the teeth are smooth, not likely you will scratch your surface material. Less final sanding needed.