If you want a notification system working separately from WordPress and run multiple websites so we developed our awesome product as a stand-alone solution. it can use Node JS & PHP multi-thread which provides 15x faster than WP plugin and consumes very very low server resources also it covers all plugin features. we highly recommend this solution for who have more than 100K subscribers where if you reach to this number of subscribers, this means that you have a big traffic and WordPress starts in this stage eats all of server resources.
Demo: https://codecanyon.net/item/desktop...3RXNk8DCqtRrzPJPuuw8sm01mIGPPzcMaAv0nEALw_wcB