Thought I'd follow up on this.
Ended up having a blood test which did show I was pretty deficient. Bloods came back much better last week and this winter has been among the most bearable in memory despite significant stressors. Ended up just taking standard OTC tablets, daily.
Anecdotally, I've slept better, improved mood, fatigue and seemed to recover more readily from training. Aware much of this could've been affected by related cofactors but I think I'll continue to take supplements each winter.
Hope this helps anyone that might be interested.
Edit: and thanks to anyone who offered advice regarding the topic