minga.io is pitched as an alternative to Remind.
simpletexting.com is mass texting platform with lots of features, which is campaign-style. I believe it starts at $25/mo and there are different plans.
duplie.com is a non-profit platform for volunteers/users which you can mass send out SMS or Email.
textedly.com is mass text platform which has a free trial. Super nice UI/flow. Starts at $29/mo.
I've helped various non-profits with technology in the past and have experience with simpletexting and duplie. You probably won't find much freemium software, since sending SMS/text costs money. Hope this helps!