Oh to response more directly to the OP...
I wouldn't just take Tec40. I would find an instructor (or mentor) doing the dives you aspire to do, go dive with them on a demo basis, and discuss your goals with them. Think about what you want to do, then decide on training and/or experience to get there.
If you want to deco dive obviously Tec40 is one possible path
If you want to assist with courses or on a charter boat then the DM course is a completely different thing...
I wouldn't just take Tec40. I would find an instructor (or mentor) doing the dives you aspire to do, go dive with them on a demo basis, and discuss your goals with them. Think about what you want to do, then decide on training and/or experience to get there.
If you want to deco dive obviously Tec40 is one possible path
If you want to assist with courses or on a charter boat then the DM course is a completely different thing...