Avoid async await Task.FromResult, plus some other minor tweaks

[Henr1k80]

Prerequisites

• I have added steps to test this contribution in the description below

There are still some unneeded awaits left from the cleanup in #16535 (was maybe in #18901)

I have also added some minor tweaks on private methods like static, use the actual type instead of interfaces to avoid looking for implementations and use async overloads when possible.

[github-actions]

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[emmagarland]

Hi @Henr1k80

Thanks for your PR to continue to avoid async await Task.Result, and some other changes, as per #16535 / #18901.

One of the Core Collaborators team will review this as soon as possible.

I don't think these are breaking changes but I will just run past HQ to make sure.

Best wishes

Emma

[AndyButland]

Thanks very much @Henr1k80 - all looks good to me, so merging in and will be part of 16.1.