Hi, I do a lot of arduino projects/tinkering, and would like to build this little tachometer from scratch. I see your original code that you first tested it out with on a 16X2 LCD & an Arduino UNO. Do you have the code you use for the 7 segment kit? I'm not sure which way I'll go yet, I've worked with 16X2 LCD's a lot, but never the 7 segment type.
One perk I see for the 7 segment display is it looks like it might be clearer to read.
I thought about using a tiny 8X2 LCD character display, and then I could have one of the two lines display either "FORWARD" or "REVERSE", to indicate the spindle direction. I think an 8X2 would fit inside the existing box in the same location.
ps. OLEDs are supposed to be very clear to read: I see CrystalFontz has this one that is almost identical in dimensions to your V2 board, so it would fit nicely inside the enclosure.
https://www.crystalfontz.com/product/cfal5016ay-module-50x16-graphic-oled-display