I decided to experiment with hardware. Some ideas are "just for fun", other should improve development convenience.

From user's POV:

• It would be convenient to upload firmware by USB, without STlink.
• It would be nice to reduce components and allow more compact PCB-s for other devices.

From developper's POV:

• Need optional tachometer, for use grinder as lab stand.
• Need some way to transfer measurements to PC.

Here is temporary link to new schematic (draft), which should replace current one when complete: https://easyeda.com/speed/ac-speed-control-v2.

What was changed:

• Direct triac drive - no more optocoupler and snubber chain.
• Use STM32F07x, with USB upload support. USB parts are still optional, if extra space savings required.
• Use MP157 instead of LNK304 - more compart and cold.
• Optional hall sensor (for lab stand). Will need magnetic label on shaft.
• Optional nrf24l01 "nano" board for realtime wireless log transfer to PC. Will use RF-nano Arduino on another end, to observe data in terminal.
• Other minor polish.

Hope to do last checks on weekend and place PCB order then.