We were in Unawatuna 2 years ago. We stayed a bit south of the main area (10 mins or so by tuk-tuk). We loved it but it is as you say very touristy but lacking in tourists. The Sri Lankans have had such a tough time. The tsunami obviously. Then the Easter bombings. We went to Hikkaduwa in March 2020 -after things had just reopened - and then Covid hit (covid curfews introduced at the end of our first week, everyone fled, we decided to take the risk and see the holiday out, and had the whole (small, 10-room) hotel completely to ourselves for another week). Not dissimilar to Unawatuna in set up but again we stayed outside the busy area. Two years ago when we were in Unawatuna was the time of the political and economic upheaval with power cuts of several hours daily, varied by area each day. Long, long, long queues for gas and many restaurants not able to open.
It just felt such a shame for such a lovely country and lovely people to be hit by so much misfortune.
Our first visit was to Bentota which is still on the SW coast but probably a bit far north for where you are looking. Even though we were in a big hotel (not the sort of place we usually choose), I recall the area away from the beach being made up of small restaurants and not too touristy. But that was a good few years ago and I expect it has changed a lot now.
I want to visit the NE coast, now that it is accessible but before it gets too built up. Wrong time of year now though.
Would be interested to hear what Mirissa is like if you get there.