I have been tinkering away in my shed quietly for a few weeks, ordering the odd bit here and there and I have finally built one of these DIY C64 Tape=>SD interfaces. It plays and records .tap files and it has worked flawlessly on all the .tap files I have tried so far.
You basically have a Arduino Nano ( I used my cheaper clone for this rather than my £25 genuine Arduino as I soldered it to the PCB rather than use a modified 32 socket ), a display ( LCD or OLED - I had a OLED spare ) which is I2C compatible and the SD Card read with Logic Level converter on board. Build up the PCB and wire up a C64 cassette port plug and then Program the Arduino and connect the SD and Display and away you go.
I have some spare PCB's and C64 Cassette connectors if anyone else fancies a go.
Here is a picture of mine - it's a bit Heath-Robinson and they are crying out to be fitted to a Case. I don't know if anyone has tried to build a A8 version yet...