There’s a reason no one puts aluminum stringers in surfboards.
I’d go with no stringer (that’s what a longitudinal stiffening insert is called in a surfboard) and a heavier layup (the layers of glass/carbon). Charlie has built dozens of boards and repaired hundreds. Trust him. I’ve built a few myself. At the thickness you are working, there’s no need for inserts or channels. The board at 4" thick is a giant flattened tube and so it’ll be super stiff on its own. And yes, those features are way harder to glass and sand.
If you want a bullet proof board, go with 2 layers of 6oz glass with another layer in a patch where you stand for the top. 2 x 6oz for the bottom and nothing will hurt that board. Simple, inexpensive, and proven in surfing and standup paddleboards for years.
If you build out of EPS foam, put in a Gore-Tex vent too.
There’s a lot to figure out on these builds, but the board construction is really simple. I’d save the innovation and experimentation for propulsion, that’s what’s never been done.
But don’t let us stop you if you really want to learn to build boards. It’s fun and mistakes are educational!
Hope this helps.