I haven't been able to find any unambiguous official confirmation online but I would be very surprised that you wouldn't be allowed to work as the spouse of a Belgian citizen once you got the relevant residence permit. I think you may even be allowed to work as soon as you applied for the permit (cf. this presentation from a lawyer working for a non-profit supporting migrants in Belgium).
If your spouse moved to the UK before 2021 and got into the EU settlement scheme then you would fall into another category and you are definitely allowed to work as soon as you applied to have your status recognised (you don't even need to wait for a permit or visa). The requirements for family reunion are also slightly different (regarding income and housing especially).
Regarding your other question, working towards citizenship, the easiest path for you would be to get the Belgian citizenship by “declaration” as the spouse of a Belgian citizen. Beside your relationship, there are three criteria for this: residing legally in Belgium, speaking one of the national languages, and proving your “integration into Belgian society”.
The last criteria is a little complicated, there are three ways to fulfil it:
Getting some Belgian secondary or tertiary education degree (presumably not relevant for you)
Getting some training and working for a while in Belgium
Following an “integration course”
So if I understand all this correctly, working in the UK would have an impact on that as it only leaves the last option open. Conversely, working in Belgium would spare you the integration course. Note that there is no requirement to have been employed for the full five years, just to have worked for a little less than a year so you have plenty of time to figure out what you want to do after your post-doc.