I’d rather not use an image, as I want the Pi to do other things aside from Nextcloud
curl installer should work if you are on Rasbian/Debian9 and Armv7
sudo apt install curl
curl -sSL https://raw.githubusercontent.com/nextcloud/nextcloudpi/master/install.sh | sudo bash
is all it would take to get Nextcloud up and running, but what
are you planning for?
Running Nextcloud required services can be bit of a strain for a RPI3b+ on its own.
Many have found rpi lacking (personal) speed requirements, and switched to other better performing hardware. But if your happy with lower performance, it does works