This is one thing I did at the start of the pandemic, especially as I came from night work, I bought a bottle of max strength Vit D (5000IU) and I take one everyday with my eggs in the morning.
5K is not bad. Ideally you should had your blood tested but with 5k daily is not so necesarry. Try to take some sun if you can, short exposures with no sunscreen and with large amounts of your body exposed. The combination will surely give you decent levels and an strengthened immune system.