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If you just want nrf52 device with battery and screen and don't mind taking it apart to get GPIOs then the Magic3/C17 is cheapest and is even nrf52840 and relatively easy to open (price is about 11EUR with shipping and taxes from aliexpress)
I wanted to ask you, I will have a look myself, but did you happen to find any unused pins on the board, like you did for the dsd6 back then?
enaon-
I found 3 pads on the back that were not listed on @fanoush's Magic3 pin table: D13 (unmarked, grouped with SXD,TXD,D+,D-), D28 (unmarked, to the far right of that cluster), and D27 (unmarked, to the near right, below and to the left of D28). I also reused I2C SDA/SCL as I have removed the touchscreen. Of course, that's the ACCEL lines as well, so you'd have to write a cooperative routine that makes sure they don't interfere with each other.
Also good for hacking is the DT28 (also cheap on AE). No "new" pins that I found, but you can find the touchscreen pads (SDA,SCL,INT,RST). When I get a chance I can check the RX/TX to see what they are...
yngv126399
DS-D6 was unique because it has two generic GPIOS on the usb data pins. Only few smart bands made by same Desay company had this feature DS-D9,DS-D6, HX03(W,F) and also SMA Q2 had it. Sadly none are available for sale nowadays. I don't have any more new DS-D6, only few used used ones (maybe atc1441 https://youtu.be/3gjmEdEDJ5A?t=6
still has some ?) , I still have two boxed HX03W.
If you just want nrf52 device with battery and screen and don't mind taking it apart to get GPIOs then the Magic3/C17 is cheapest and is even nrf52840 and relatively easy to open (price is about 11EUR with shipping and taxes from aliexpress)