You are right, yes, it is a lot of work. This is what TAC gave me back in December 2020 and I did it few times and works really well. Again, not a script, but I like this method.
--->To add address-range via API:
mgmt_cli add address-range --batch address-ranges_full.csv
#cat address-ranges_full.csv
name,ip-address-first,ip-address-last
range1,10.0.0.0,10.0.0.100
---> To add a network via API:
mgmt_cli add network --batch networks.csv
#cat networks.csv
name,subnet,subnet-mask
network1,10.10.10.0,255.255.255.0
network2,20.20.20.0,255.255.255.0
network3,30.30.30.0,255.255.255.0
---> To add a host
mgmt_cli add host --batch test.csv
#cat test.csv
name,ip-address
obj1,192.168.1.1