You are right there is as yet no guidance on the timing of Covid vaccination and steroid injections. We are all hoping that something will be issued from the BSR or the British Pain Society to clarify this situation (and perhaps more urgently regarding steroid use and immunosuppressants in general)
Obviously none of us would wish to do anything which would hamper the effectiveness of Covid vaccination which seems to be the priority here obviously. And there is some data available for the different vaccines detailing how long it takes to build up an immune response post vaccination.
(You may be aware that PCRMM however did some work looking into steroid injections and flu vaccination recently. The available data suggests there is probably no effect on the response to flu vaccination from the dose of steroid used in joint injections.
The paper you mention was included in the searches. However that paper is retrospective, they include search terms such as “influenza,” “probable influenza,” or “influenza like illness.”, and there is little detail regarding the timing of vaccination or injection. So it was not felt to be a good paper to draw firm conclusions, only to suggest further research perhaps)