Hi Su,
Thanks for reaching out. You will be able to use the AdwordsUserListService.mutateMembers() to ADD or REMOVE directly from a user list without removing the entire list of members and then re-adding them. Please let me know if you have any further concerns.
Thanks and regards,
Xiaoming, Google Ads API Team
Hi Su,
As for the mutate in Adwords API, each mutated item counts as one operation and the daily operational limits are 10,000 operations for "Basic Access" and unlimited operations for "Standard Access". Please refer to this documentation for more details.
Thanks and regards,
Xiaoming, Google Ads API Team