I'd please suggest the ability to keep the Nimble Outlook Add-in 'permanently open' rather than having to click it for each email that is read or sent.

Microsoft appeared to allow this capability with a Pinnable Taskpane feature release, though I'm not sure it's available for Office 365 and Outlook 365 Desktop
https://docs.microsoft.com/en-us/archive/blogs/exchangedev/pinnable-taskpane-in-outlook-2016