Bram, Using what you have on hand is a good choice.
If your lumber has edges rough enough to cause problems riding the saw's fence you could attach a piece on top to act as a guide edge. Keep this away from were the blade will be cutting. This will produce a straighter edge for the fence and the piece with the guide will also have a good edge and can be cut along the rough edge to clean it up.
In the day of Roubo, there was a much better availability of large slabs of wood to build a singe piece bench top. There are not many trees left to produce such slabs of that size today. Many of them are either protected by law or the wood is going to be pricey.
Likely the worst that will come of your plan is you will find things you want to try, motivating your desire to build another bench.
One thing to be thankful for is you don't have to do it by hand:
https://sawmillcreek.org/showthread.php?167535
Do to so many other things getting in between me and time in the shop, this project is still on hold and may be abandoned for another method.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)