Alternative for "so" as conjunction

hence, since then or now, in a strictly temporal sense, rather superfluous.

In the example, so is not a logical conjunction. I do in fact not see an unambiguous logic behind the sentence. You could as well ommit any conjunction and have the same result, as far as the temporal correlation of events is concerned.

The eistemic implication is downright paradox: You have been affected, so it appears you generally aren't careful enough (which might be inevitable, and thus inevitably true, no offence).

That said, one might infer that you have taken notice and became so much more careful. But that's an odd, contracted way of saying it. Adding "therefore" does not help much at that, because the imolication is still embedded in the unwritten context.

Myself and most of my colleagues have been affected by the coronavirus pandemic, so we're extra cautious.

If you only want to replace 'so' in original sentence by an alternative conjunction:

Myself and most of my colleagues have been affected by the coronavirus pandemic. Therefore, we're extra cautious.

(Instead of 'coronavirus', I'd use a more specific term. The term 'coronavirus' describes many viruses that share certain characteristics. The WHO name of the virus causing the current outbreak is COVID-19.)

I'd write your sentence as: 'Most of my colleagues and I have been affected by Covid-19. Therefore, we are now extra cautious.'

If you want to more closely associate the two ideas of the COVID-19 virus and the precautions, you could write: Most of my colleagues and I have been affected by COVID-19—we are now extra cautious.