Hello Dylan, fellow designer, fabricator and machinist! Thanks for posting your work and I’m sure its been an awesome experience integrating cnc into your workflow. Definitely keep us updated on your BMW EV project. Question. Have you ever welded a crash and reran toolpaths? Its an interesting spot to be in when you can tig and magic erase things.
Big Stock - A material where the thickness exceeds flute length. There’s a whole theory on part design, tooling, cam and workflow when it comes to big stock machining. When you are forced to use increased stickouts, its very helpful to use a force calculator like the @gmack
You also don’t have to go slow all the time, try separating the toolpath in half. Start with one that has the minimum stickout on the tool and maximum flutes and run it to max flute depth. The workbook will let you know the deflection and machine forces so run it hard. Then switch to a lower flute count tool with longer stickout to finish the bottom. For example, a single flute will minimize torque needed in the cut which will minimize machine forces and end mill deflection. That directly translates into surface quality, tool life, and chipload you can run. Finishing requires a light touch as well.
It looks like your part is lower than the stock height which means you could float it with tabs. Another way to look at making a tophat like that fast would be to use a vise with end stop or make a fixture plate with a corner reference. You could band saw your stock for a single part per piece workflow and perform a flip. This would allow you to run the most aggressively you could and machine all sides which both are benefits. Plus a repeatable setup helps if you ever run into problems.
Now don’t get me wrong, nobody loves upgrades more, and linear rails/spindles will help to some degree. But everything starts at feeds and speeds, and the move V wheels you have, the more it matters. Good call on the aluminum bed! Definitely get a vise and some consumable fixture plates setup on there. A nice little 3 jaw chuck can come in really handy as well.