Isolated Power Supplies for Inverter Gate Drivers
Should I wound 1 big 50 Hz step-down transformer (380 VAC to 24 VAC) with 5 separate secondary windings...?


No. There are at least two alternatives that will likely be less expensive.

One possibility would be to convert 380 V, 50 Hz to a higher frequency and use that for the primary of a single transformer.

Another possibility would be to use 5 individually isolated, 380:?? V SMPS's.

In order to make a valid decision, you need to estimate the costs based on your anticipated manufacturing location, volume etc.