Another vote for Textra, or Pulse. Both are very customizable, perhaps Textra a little bit more so and its interface is more refined, Pulse being new and all. However Pulse incorporates texting from a browser. Before I used to use Textra + Mighty Text.
Mighty Text is more feature-rich than Pulse's online texting but that could be a good or bad thing. If you just want text messaging from a computer, Pulse is to the point.
Anyway for customization Textra and Pulse are really good, Textra being more polished, but Pulse just gives you the bonus of web based texting as well in one package.
No idea what the OP's aversion is to third party apps, especially well-established ones with a great reputation, they can make or break a platform actually.