One of the unique features of MacBook devices, is that as of the 2019 16", they can charge off of as little as 5V. I have powered mine from a power bank and cable that did not support "fast charging" (I think it topped out at 2-3A) and the computer charged just fine off it, albeit exceptionally slowly and barely at all when running.
If you are curious about what works with what, there is a YouTube channel called ChargerLab that tests different devices plugged into different chargers.
Yes, you can get by on significantly less power if you can charge it while the lid is closed. It might just run on the USB-C power with the lid open, and say "not charging" but when the lid is closed it will start to fill up the battery to capacity.
Unless you have very stressful workloads, you should be able to use the laptop normally with a lower capacity power brick. It will just charge slowly or not at all. Absolute worst case you can close the lid to charge it.
Do macs let you trickle charge at lower power? If that works, you could limp along with any USB-C charger.