If you want free service and you post once a day I recommend Later. If you post more than that there's a paid plan.
You can connect each account individually, so you don't have to pay the option for multiple accounts, although it's kind of a pain in the a** to log in, log out of each account, but since it's one day in a month where you schedule the posts contrary to now where you have to do it every single day, I think it's pretty much worth it.
P.S. I am in no way, shape or form affiliated with them.