As someone having deeply commercial interests here... it's difficult to comment without suggesting all I want is your money, but I'll do it anyway!
As you are so far down the steep learning curve of routing and networks, I suggest you go on some Mikrotik training courses or pay for an MT consultant to help you through this or pay for one-to-one tuition as he takes you through the network design process, helping you with the hardware purchase decisions, teaching you how to implement your desired network correctly. Someone with skills in Mikrotik and Ubiquiti might also be wise.
The topic of routing on a WISP based network is not Mikrotik specific, but diving straight into OSPF routing without an understanding of what you are doing is asking for a wasted and stressful time as you battle with a system you can't control as you don't really understand what you are doing. You will then start to wish you had never started, or worse still, blame Mikrotik for a non-functioning network, with your customers getting upset with you more and more until you (or they) eventually give up, unless you are lucky that you can learn as your customers suffer from your mistakes?
Adding OSPF to a working L2 bridged network and converting it to a fully routed solution is trivial for someone used to doing it. But have you got the time and will you get everything you need to know by asking ever more and more questions on a forum, when you really need that time to run your network and actually buy in help on a one to one basis to teach you what to do for yourself so that when it's all up and running, you know how to continue to maintain your 'new' network as you will have been shown how to, using your own network?
Might also be wise to buy a few routerboards (450g?) and connecting them up as a test network, so any trials you perform, while you learn routing, are done on a network that is sat on the bench and it does not matter when it all falls over.