Holding buoyancy and trim is a basic open water skill.......adding tec skills shouldn’t affect that within reason.Tec40 is an INTRO level course. No course can ever teach you expert skills; the course only gives you the knowledge of how to do the skills and for you to then go off and spend some considerable time practising and perfecting your skills.
It doesn't matter if it's Tec40 or Fundies, they're the same outcome: adequate skills to go off and practice.
If only the marketing material would make that clear. If only the local dive shops would exploit this need for continuous skills training for excellence.
What should be the case is that as you progress to the next level, the entry qualifications are higher. In the case of PADI that's Tec40 --> Tec45, TDI it's Intro to Tec --> Advanced Nitrox & Deco Procedures, GUE it's Fundies --> Tec1.
When you arrive at the next level you must be able to demonstrate competent skills at the previous course level. Therefore you simply cannot -- unless you're an utter diving prodigy -- go immediately from one course to another without spending (lets say) a few weeks practising and refining your skills. This, IMHO, is where PADI does go wrong. Tec skills aren't like moving through OW to AOW to Divemaster. You need consolidation time so that sub-standard divers are either improved or weeded out of the system before they kill themselves and hurt others.