6mo ds2 is fully breastfed and never had a bottle as I'm rubbish at expressing milk. Tried it all...hand/manual/hospital grade electric pump and only get 60ml max per session. I was the same with ds1 who ended up being mixed fed.
I really want to go out for the day with ds1 and do something just us but will probably miss 2 feeds. I have enough supply in the freezer and I think ds2 will take it in a cup. Will my milk supply be ok missing 2 feeds?
I was rubbish at expressing with DC1 and not much better with DC2, so you have my sympathy.
I think at 6 months your supply will be fine missing 2 feeds although you might want to express at least once, just for your own comfort. I'm not trained though so someone might come along soon and correct me!
If you can't take a pump out with you, then you might want to express some off by hand expressing.
DS is approaching 6mo and like you I can only usually get 60ml in one pumping session, unless he's slept longer overnight and therefore I am a bit more full in the morning, in which case I can get about 100ml.
One day away isn't going to mess up your supply I would think. If you are going to be away for two feeds or more you probably will want to express for comfort (but it doesn't need to be a full express IYSWIM? Just to get you comfortable). Bet you'll want to plug baby in as soon as you get home though!