if the 10,000 people you want to send are arbitrary and irregular, it is inevitable to generate a list of 10,000 people, whether manually selected or stored, at least once. However, if it is regular, use a method similar to the permission to read the post. Every time a user requests a notification list, it can be checked whether it meets the requirements. It can also be divided into user groups, and users in some groups can view a notification.

I think your idea will be a bit troublesome to use, but the effect is the same as storing a list directly. If you have to store this list, you might as well categorize it, enumerate it directly if the target users are sparse, and use your method if it is dense. Of course, this may require two fields.

you can try saving the user list into the structure type to become
id
message_id information ID
user_id_list user ID list

this is based on the number of users. If it is only 10000, you can use your method. If the amount of data is relatively large, 1 million 10 million, you can only use the first method. Even if the amount of data is large, but there are few fields, the query is still very fast.

all data can be stored in redis
queue Synchronize to mysql
mysql can be divided into tables

this idea can be quickly realized by using user role + subscription function without having to send it thousands of times.