I would say please do consider an Azure SQL DTU Model S6 or above. Use Premium tiers (P4 or later) if you want good IO storage performance.
More suggestions, please consider do the data changes (that occur once a day) in batches not in just one batch. After those data changes, try to defragment indexes and update statistics and you will get the performance you are looking for.
Check out the dynamic management view related to missing indexes as missing indexes on big tables can make you scale tiers on Azure SQL PaaS or face poor performance.
Bottom line, I think you are missing to do daily database maintenance and take care of some possible missing indexes and query tuning and you will be fine.