On F-Droid thereβs an app called Multi SMS, that should do the trick and I have not stumbled upon any ads so far.
You can send SMS to various people, selecting from a list and you can create groups as well. The settings menu (three dots) is in the lower right corner.
When selecting to send to a group, you first get the info that there is no group. Tapping on the three dots lets you create a group; assigning a group name and selecting members from a list.