For small microcontrollers you can just buy modern radiation tolerant chips from e.g. microchip for about $2000 or so per unit, but you need a certain expertise to select the other components you need and design/qualify. Or you can any one of dozens of different cubesat cpu boards, which typically have COTS components and has gone through radiation screening (which is way more work than "nuking under a beam"). These are typically sold for cubesats, but don't meet the requirements for qualification and documentation that is required for an ESA or NASA-related project beyond a cubesat.
There are the famous "old as hell" rad-hard powerPCs which cost $200k, used for historical reasons or extreme environments like a fly-by of Venus. It's true that older chips may be inherently more radiation tolerant due to larger features on the silicon but it's a misconception that CPUs used in space are therefore always old as hell. These are usually selected because they are good enough and to reuse design and qualification from a previous project instead of spending millions and ten person-years on a new design.
But commercial spacecraft these days usually use something like this[1] triple-redundant 50MHz softcpu implemented on an FPGA or this[2] Ultrascale ~1Ghz SOC protected by a secret sauce of separate FPGA for watchdog and overcurrent detection. Both are modern, run linux and use modern development environments/tools. The former is slower but can recover upsets in realtime, while the latter is much faster but needs to reset itself to recover at least some types of upsets.
[1] https://www.aac-clyde.space/wp-content/uploads/2021/10/AAC_D...
[2] https://xiphos.com/product-details/q8