I use dd to write the image to an SD card. The card is not the issue. Plug the card in and turn on the pi, enter WiFi, Keyboard, Language, Location, and wait. How long should it take? Is this related to it not expanding the SD Card memory prior to downloading updates?
I have successfully used dd to install FRCVision, Raspbian, Ubuntu 20.10 Desktop, Ubuntu 20.04 Server, on Pi 4's and Pi 3's using dd on linux. Expected Ubuntu-Mate 20.04 image to work too.
Does Etcher or BelenaEtcher resize the card after installing the O.S.? If so then the instructions at raspberrypi.org needs to be updated.
http://www.raspberrypi.org/documentation/installation/installing-images/linux.md
On linux
df -h
To list hard drives and find drive letter for your sd card on liux.
sudo umount /dev/sdXY
To unmount all the drives on the sd card so we may overwrite the image.
sha256sum imageFileName
To check sha 256 sum and verify downloaded file is not corrupted.
unxz imageFileName.img.xz
To remove xz compression on the downloaded image file.
dd bs=4M status=progress if=imageFileName.img of=/dev/sdX oflag=direct
To write the image to SD Card