Yesterday night I tried uppdating to 17.3.1 from 16.6 for my ipad mini 6 and found similar issue where it was connected to wi fi router ( 5 GHz band) with valid IP, was in charging (less than 100%), and storage is around 12 GB free. The update required 3.5 GB space. Multiple restart did not help.
This morning after going through this forum I tried switching my connection to a 2.4 GHz band just to get another IP though the last one was working fine.
Post connecting to a different ssid in the same router now the issue is fixed and update has started. I believe 2 points can be considered here, It may not be an issue with 5 GHz band at all and refreshing the IP might have worked alone. Secondly, when the udate started it was 100 % charged which was not the case earlier. SME’s may answer better in this cocern however, the issue is resolved the way I described above.