The error in the previous attempt was uncorrectable to me. So I made some design changes: the three issues from the previous attempt, plus added the pinout table for the ISP.

New order, one week waiting, board arrived:



Built it once more.
There were some numbers on the display, but it didn't work as expected. I realized two more design errors:

• The display anodes were swapped so the number was readable from right to left
• The LEDs didn't lid - It was rotated by 180 degrees

Rewrote the code for the display, desoldered the LEDs and added new ones rotated (those LEDs are unable to withstand the heat fro the heathgun)
Now it is working as expected.