Do not be sorry for the post, read everything, you have been heard.
A committed guy does not flirt.
Girls do not send "i love you" and "shower" texts to a guy they are not intimate with.
He lies openly to you while you read the message exchanges. And he is not on Tinder for job interviews.
Even if we say "He just lies to the other girl about being in a relationship with her because he is really with you" does not make it any better (by the way, want to bet that someone like this would say the same to the other girl?).
I am sorry you are facing this stuff, trust your instincts, not the words.
He seems really the kind of serial cheater for what I read
If you read the stories here you will see these patterns of lying, inconsistent excuses, it is always the same pattern.
Preserve your emotions and your peace. Confront him, you need clarity, you seem involved, but you spotted already red flags.
I think your instincts are right, do not suppress them, do not trust, lies from cheaters are very weak but he will gaslight you as you were crazy. Then it will collapse.
We are here to support you