Hi Joana,
You don’t say where your final destinations are, so it’s difficult to give tailored advice. However, if you use an Oyster card to PAYG then definitely add your railcard to it. You will get cheaper off-peak fares, and even though you’ll pay full price in the evening peak on weekdays, the discounted off-peak cap will limit the effect of that. You can’t at the moment get railcard fares when using contactless on bank cards or mobiles.
Depending on where you need to go to, Hendon could be a very good place to switch as it’s on the boundary between zones 3 and 4. You will need to get off the train and use your ticket from St Albans to exit, then use your Oyster card to enter again. Let me know the places you end up in and I’ll see what specifically I can suggest to keep the costs down. If you can give rough times of journeys involved too (both to and from work) then it’s even better.