A couple of considerations here beyond the feeds and speeds and tabs:
- cutting a slot as narrow as the endmill results in near constant 100% tooling engagement — best practice is to add geometry and cut as a pocket down to tab depth at least: Adding geometry to cut as a pocket with a finishing pass
- leaving a roughing clearance and taking a finishing pass will help with final dimensions
- if you haven’t calibrated for belt stretch, doing so will help with the evenness of parts: https://docs.carbide3d.com/shapeoko-faq/how-to-calibrate-the-machine-for-belt-stretch/