As a database admin, using internaltmpmemstorageengine=TempTable (default), I would like the temptablemaxram size to be adjusted to the plan and not use the default value of 1G.

When using the new default TempTable engine for temporary tables the setting temptablemaxram=1G (default) controls the max. amount of memory for ALL temporary tables.

"The setting temptablemaxram controls how much is available to ALL temporary tables before converting to an on-disk table. Looking at our 50 queries example earlier, say each one needs 256MB of memory. With the default setting of 1GB, the first 4 queries would be stored in memory, and the remaining 46 queries would immediately go to disk as InnoDB temporary tables."

Source: https://www.percona.com/blog/adjusting-mysql-8-0-memory-parameters/

An application with heavy use on tmptables currently should use the internaltmpmemstorageengine=MEMORY setting, as this will allocate max tmptable_size=256M to each query connection.