People here seem to mostly discuss the legacy backends the the nice newish ODK Central, but what about some other modern ODK server backends?
https://github.com/qlands/FormShare for example looks quite nice, with the main features being extensibility though plugins and longitudinal survey support. The public free instance works nicely, but I had some trouble getting it to work on my own server (elastic search is too heavy on server resources, and it seems to absolutely need MySQL and not MariaDB). All it all is seems like a really nice modern alternative to use ODK collect with.
Another slightly older option is Nemo: https://github.com/thecartercenter/nemo
I have not tried running it on my own server yet, but it comes with a build in form-builder and also has SMS support for simple notifications and surveys. Anyone has experience with it?
Ah, and both support collaboration via projects/missions and have ODATA support. Formshare even has bi-directional ODATA that allows one to clean submission data directly in Excel (with a plugin).