Every time I start Obsidian, it tells me that an update is available. Specifically a major installer update is required and I need to manually download / reinstall Obsidian.
You may have tried this, but I suggest that you check your installer version, which can be different than your current version. The installer version, when needed, has to be downloaded from the website. Whereas regular updates can happen automatically within Obsidian.
The easiest thing to do would be to download Obsidian from the website just like you did the first time you installed it. Hopefully this helps.
Are you saying that re-downloading Obsidian and doing an installation on top of my existing install is different to the auto-update process that Obsidian does by itself? That type of behaviour would be new to me.
I must say this behaviour seems counter-intuitive. If the displayed version of my running software is the same as the version for download, my software is up to date.