Two Software-as-a-Service solutions have already been mentioned:
https://mysites.guru/ (formerly myjoomla.com)
https://watchful.net/ (formerly watchful.li)
And those commercial SaaS solutions are both good.
However, I prefer a self-hosted solution. A few years ago I started using YourSites.net (commercial extension) and I really like it:
You install the ...
When you are in the Joomla Update page select Options up in the top right corner.
From there change the channel to Joomla Next, you are probably still on Default which is Joomla 3.x
You may also have to go into Extensions->Manage->Update and Clear Cache and Find Updates after switching between the Default and Joomla Next channels.
Joomla doesn't support updating thru SFTP. Joomla only supports FTP.
There is a major difference between SFTP and FTP.
FTP (File Transfer Protocol) is a plain text protocol using 2 separated connections for transferring data. It also doesn't support encryption. Except if you use FTPS which is an addition to FTP. It needs port 21 (default) and a data port.