SD Card Adaptor

Numerous USB-c card readers here:
https://www.amazon.com/s/ref=nb_sb_...&sprefix=usb-c+sd+,aps,197&crid=10LVSSV44P8VQ

If you don't mind "just a little larger", there are some that offer USB3 (A-type) ports as well.
I saw one by G-TING that had 2 card slots (SD and mini SD), 2 USB ports, and an ethernet port also.

What kind of external drive you buy for backup depends on how large your INTERNAL drive is.

For 256gb, I'd get an equally-sized USB3 SSD, and instead of Time Machine I'd use either CarbonCopyCloner or SuperDuper to create a bootable cloned backup. All the files will be there, and it's a SECOND bootable drive if you get into an "I can't boot!" moment with the internal SSD.

 

I bought the Sandisk UHS II USB C card reader from Apple. Ive been very happy with it.

https://www.apple.com/shop/product/...reme-pro-sd-uhs-ii-card-usb-c-reader?fnode=8b

 

I have the Satechi USB Drive looking one and it works well. Has both the smaller and normal sized SD Card reader on it.

 

yeah and I don't know why. Ive used it consistently everyday for six months and with several different types of cards and I've had nothing but success with it.