There are many stm32 variant boards, like the BluePill, but the BluePill is the cheapest.
For more horsepower there are a variety of relatively cheap STM32F4 boards that are used by the deone community - search for STM32 Flight controllers. I think I have seen these close to $10.
I Mostly work with Orange Pi’s most of the cheap ones are based on Allwinner H2+ and H3’s, they are far more bang for the buck than Raspberry Pi’s.
The OPI0 is much more useful than an RPI0
It has taken several years with no help from AllWinner but the Linux Kernel support for AllWinner SOC’s is now very good.
And you can get an OPI0 with eMMC to boot off of which eliminates SD Card failures
Buying in quantity and getting reasonable shipping is an issue.
I have not looked at trying to buy 5 OPI0’s at one time.
The shipping is still exhorbitant.
But I have designed projects arround OPI0’s.
Remember you get ALOT essentially for free with any form of Linux SBC that will cost alot of programming effort on an STM32 or Arduino.
You have far more Ram and Flash and performance, and a variety of peripherals on board.
Quite Often I redesign Arduino or STM32 boards to use an OPI0 and often save enough in other parts to make up for the more expensive board.
It is much easier to do IoT with a linux board.