How to hear "Do"?

Bare with me. I'm 30, and I've been singing in a Barbershop choir for three years. We do alright, I sing lead/baritone and our choir of 15-20 people took second in district. I'm just learning how to sight read, just got off of mnemonics for treble/bass clef, and have been using the app Sonofield for almost a year, which is an ear trainer that plays tones over a drone and has you identify which scale degree is playing.

I've gotten decent at Sonofield and my intonation has improved immeasurably.

However, now I'm realizing, when I'm in the middle of our repertoire, between the constant chord changes and Barbershop chromatics, I do not know where Do is for most of the time, so the ear training that I've been doing to figure out which scale degree I'm on is not kicking in. I'm realizing I don't have an innate knack for knowing where "Do" is when I'm listening to music, too.

So... yeah. Wondering if anybody has some tips while I dive into this problem. Do I just need to be more intentional and try to hum a Do to a learning track? Any tips would be appreciated! Thank you :)