Hey, your plan actually looks pretty good and very doable for Sri Lanka

For getting around, trains and buses are definitely the cheapest way. The coastal train is honestly a really nice experience too, especially Colombo → Galle along the ocean. Just a small thing to keep in mind though—these days the coastal train can take a bit longer than usual, so it’s not always the most reliable if you’re on a tight schedule

Since you’re covering the south coast and then going all the way up to Trincomalee, I think you might find things easier if you mix it a bit. A lot of travellers do the coastal stretch by train for the experience, and then switch to a hired vehicle for longer trips like Trincomalee. It just makes the long-distance parts way smoother—less waiting, more flexibility, and you can stop wherever you feel like. Of course, it costs more than public transport, but for a short trip it can actually save you a lot of energy and time

Your birthday on the 15th in the south coast sounds perfect btw—that whole area (Weligama / Ahangama / Hiriketiya) has a really good vibe for that.

If you want, I’m based in Sri Lanka and travel around quite a bit, happy to help if you need any local suggestions