Sql-server – Want to get executed query users log in SQL-server

The first thing you'll want to do is make sure there is an alternate key (usually via a unique constraint) defined in the taskmembers table, that consists of the task id and member id. This key will drive all data manipulation, and the surrogate key won't be involved at all.

Once you have that key in place, I suggest using the MERGE statement to perform data manipulation. This will avoid having to delete a subset of data and re-insert it. Instead, by matching the source and target data based on the alternate key, incremental insert/updates can take place. The MERGE statement also allows you to delete from the target where records don't exist in the source (i.e., a member of the task was removed).

The DELETE/INSERT pattern is really wasteful of resources because of the extra amount of logging involved, and the unnecessary locks that are required while the data manipulation occurs. If this is a "hot" table, this kind of pattern can easily lead to deadlocks (if not using snapshot isolation), and as you're finding out, wasted key space usage. With the MERGE pattern, new keys would only be required when members are actually added to tasks.

Data is bulk inserted into these tables in the order of ~ 200k rows per day... out of ~5 million rows

That's 200k new rows that do not have any stats for, so querying for those values will make the optimizer come up with scans because of the outdated stats do not inform it that the new values are in the range sampled. Force a stats update after the bulk insert.

Not sure autostats can help given that 200k inserts fall bellow the 5 million 20% cadinality trigger, see Statistical maintenance functionality (autostats) in SQL Server.