10.11 through 10.14 (Mojave) list minimum RAM as 2 GB, which is what your MacBook Air has.
10.15 Catalina lists 4 GB minimum.
I have access to an iMac from about the same time, running Mojave with 8 GB. It's usable, but not ideal. On my home system, Mojave tends to use 10-11 GB (out of 64).
I'd say it depends on your storage. If you have an SSD, I'd try Mojave. If you have the HD option, upgrade - SSD prices have plunged. Let the memory overflow to the SSD. If it doesn't work out, High Sierra can read/write APFS volumes (for sharing external drives), and I was happy with it for a long time. Not a bad choice IMO.
Backup your current install first, in case Mojave is unworkable. So you can install an earlier OS and restore everything. Migration Assistant isn't so good at going backwards.