It is important to have a logo / more distinctive branding on the site. I would also consider some UI updates. The color palette is one you should seek to improve, ideally through some A/B or multivariate testing. It is not clear (at least not at first glance) what conversion goals you are targeting, and what call to action you are hoping site visitors to take. It is very important to define those metrics before investing too much more time and effort on site enhancements and testing.

Total Addressable (at least in the US) can be built based on market data sets, both free and for fee. For early child care centers, Market Data Retrieval and NCES (publicly available data set) are two recommended sources.
For Serviceable, that depends on a) your own business / operating model and b) the current and projected penetration of comparable competitors in market. For the former, consider things like technology usage and implementation capabilities. For the latter, if you have identified clear competitors, it is unlikely that you will have access to accurate info. So, if you can estimate these numbers and reverse-engineer, that's a good starting point. This would also be a great time to consider how your SAAS product is differentiated and will give you a chance to think through different scenario models for customer "switching" (from an existing competitor product to your own).

I won't address the factors common to all digital startups here, and focus on factors that might be specific to education, and B2B in particular.

* Understanding the Sales Cycle - it's different in education, particularly K12 and HE (at least here in the U.S. market). It's important to plan revenue and projections, accordingly. The time to move through the entire sales cycle can be longer than anticipated (if you haven't had exposure to K12, for example).
* Understanding the Audience - who is it for? and who are the stakeholders? Like most B2B plays, you are likely going to interact with multiple audience types / personas - define them and be intentional with your approach towards each (whether it's via sales, customer development, implementation support, etc)
* Articulating Value - there are many ways to define value in education. Some of these are clearly financial in nature, but many, many are not. If you're building something aimed to increase levels of student achievement, teacher efficacy, etc, then think about to position your product / service in the right way.

Hope this helps and is somewhat on track.

The fact that you have revenue is a promising start. If you have some recurring revenue, you have a business. If your "renewal" cycle hasn't yet hit, that will be a key inflection point. In the meantime, continue to speak with your customers about the value they are getting from your product / service. That's very cheap and shouldn't incur any steep opportunity costs.