You can go with Sendinblue. They also offer free plan where you can upload unlimited subscriber and can send 9k emails per month.
Just about all ESP's will let you upload your list. Many will let you upload without the users having to be sent a confirmation email. The main thing is to make sure the data is clean. If your list has excessive bounces or users on a blacklist it will get your account flagged and possibly terminated. How many subscribers do you have and when was it last mailed?