I would not put OLEDs for the reasons you mentioned, but also because it would drain the battery in 1 day vs 1 week.
Steel is likely better and surely stronger, but manufacturers charge 2x more for steel alloy machining. At that price level, titanium also becomes an interesting option.
Yep 150 mAh, it's what I had on hand. But I should have space for 300-500 mAh. I recharge the left (master peripheral) once per ~1.5 weeks. Right side only needs recharging every 2–3 weeks.
Final product, i.e only the parts and components were $400.
The cost of all the tools and software I did not track but I would guess I invested somewhere around $1K which includes software licenses, shipping, new tools etc.
But I find tracking the cost of research a bit impractical as it doesn't include indirect costs like billable hours that I technically could have spent on clients etc.
The PCBWay order came from China to my EU address, I wasn't aware they have manus in other places. For a set of two enclosures I paid $187 excluding shipping.