May I just add that OTAs (like Agoda) is able to book on Thai Lion Air as well. You can use Google Flights to find the cheapest option too, if you haven't already.

I don't think there really is any major benefit to booking direct when it comes to domestic flights in Thailand, the stakes are quite low.
On my most recent SL flight, I was able to select seats and pre-order the extras despite booking with OTA. The same cannot be said with carriers like Thai VietJet, however.

In terms of cabin comfort, I'd say if you're a relatively tall person, the additional legroom seat is totally worth paying extra for.
Cabin crew is great and attentive, you can also ask them to switch seats if you somehow got randomly assigned a middle seat. They won't bite you.
Pre-ordered meals are not great (both in quality and variety), I like their competition better.
Planes are quite new, they've been operating since early 2010s I believe and so you won't end up on planes that are too worn out; I also like the B737 overhead bins better than A320s.

It's your typical ULCC experience. You get what you pay for.