How to Limit DTU Percentage Consumption for a Given Session in Azure SQL Database?
Have you considered using the Resumable Online Index Rebuild feature so you can schedule X executions of one minute of those indexes?

ALTER INDEX [ix_CustomerIDs]
ON [ContosoSales].[ConstosoTransactionData]
REBUILD
WITH (ONLINE = ON, RESUMABLE = ON, MAX_DURATION = 1 MINUTES);
GO

ALTER INDEX ix_CustomerIDs ON [ContosoSales].[ConstosoTransactionData] PAUSE

ALTER INDEX ix_CustomerIDs ON [ContosoSales].[ConstosoTransactionData] RESUME


Have you considered to scale up the tier to Premium to speed up things when performing index maintenance during any small maintenance window? You will pay Premium for just the hour where you scaled up and users may perceive less performance impact.