> Also, is it viable to produce something like the A320 in low quantities when airlines need fleets in the hundreds?

Bombardier thought so, and did so! They developed and introduced the C-series aircraft to compete with smaller 737s and A320/A319. After introduction, Boeing fucked them over so hard that they sold the aircraft program after introduction to Airbus for a token sum. Airbus now builds and sells the C-series as the A220.

https://en.wikipedia.org/wiki/CSeries_dumping_petition_by_Bo...