Trying to plan ahead so any know if card payments are common at hostels in DRC (DR Congo) or cash is king?
I'd be very cautious about relying on card payments. The vast majority of hostels are not equipped to take cards. The best plan is to have a mix of Congolese francs for small purchases and a good amount of US dollars for your hostel payments and other daily expenses.
Don't even think about using a card for your hostel stay. I stayed at a few hostels and small hotels in the DRC, and not a single one accepted a card. Even at the main banks, getting cash from an ATM can be a bit of a challenge with a foreign card. You will definitely need to bring US dollars to exchange or use a money transfer service if you need more money.
Cash is absolutely king here. I was just in Kinshasa a couple of months ago and tried to pay with my card at a guesthouse, and it was a complete no-go. The place was not set up for it. My advice is to assume that you will not be able to use your card for hotel. You'll need to go to a bank to get Congolese francs or US dollars to pay for your stay.
You're going to need to have cash on hand. The vast majority of hostels and guesthouses in the DR Congo, even in the capital Kinshasa, do not accept credit card payments. The financial infrastructure for cards is not widespread, and most businesses, including hotel, operate on a cash-only basis. The local currency is the Congolese franc, but US dollars are also widely used, especially for larger purchases.