I'd echo what others have said about the benefit of seeking out some form of preparatory training and assessment prior to enrolling on a tech program. It's important to understand the tolerances that you'll be expected to perform at in training - and to have improved your core skills to meet those tolerances in advance of the course. Where possible, transition onto your chosen technical rig in advance, so that equipment familiarity begins to get ingrained.
I personally feel that insufficient emphasis is put on this by many agencies - and, consequently, technical courses generally tend to focus on 'remedial' development of buoyancy, trim, propulsion, situational awareness and a precision approach - rather than being entirely focused on developing the new procedural and equipment capabilities needed.
Whilst many tech instructors carry out some form of base-line assessment/check-out prior to courses, there often lacks any formal performance requirement. GUE/UTD solved that dilemma through the 'Primer' and 'Fundies' courses. Other agency instructors might offer mentoring or
clinics as a preparatory basis (although these won't be formal pre-requisites with the agencies they represent).
IMHO, far too many technical instructors accept students on to course/s without any knowledge of student baseline capability - this is especially true where students desire to progress through multiple courses (i.e. 'zero-to-trimix') in a single, unbroken, phase of training.
My advice is to concentrate entirely on developing the core skills needed to technical dive - and ensure you have those at a sufficient level of comfort and level of precision, before engaging on formal technical training. You'll get much more out of training that way - progression rather than remediation.
Having covered the basic pre-requisite capabilities, you need to do a lot of research on your potential training provider. I believe that the instructor is more important than the agency. Some agencies are more robust in their instructor standards than others - so there is some justification in expecting a 'lowest common denominator' of instructor capability from X, Y or Z agency. That said, if you are properly researching the potential instructor/s, then the agency 'minimum standards' cease to be an issue.
As others have said, expect to be humbled and frustrated during your training. That's a good thing and is evidence that you're being trained properly.
Don't try and rush through multiple training courses, in a short space of time. Most divers are unlikely to fully assimilate a full spectrum of skills and procedures into their muscle memory, at the correct standard and precision, without substantial post-qualification practice and repetition. Allow for some time/dives between formal courses for you to turn 'knowledge of skills' into 'assured capabilities'.
Technical diving demands huge investment and commitment. Many people focus on the financial implications of that... and aren't wrong to do so. However, the investment and commitment of TIME is also critical. Be patient and don't attempt to 'fast-track'... get the most value from training through a staged, progressive development of capability over an extended timescale.
As a technical diving instructor, I am actually of a split-mind over the relative merits of having a single instructor versus multiple instructors. When I teach, I am confident of my ability to mentor and develop a student through multiple phases of training. I see a huge benefit for that. However, I also see that exposure to multiple approaches, opinions and mindsets is also greatly beneficial to the long-term development of a technical diver - and this can be achieved through seeking out a variety of competent instructors as you progress. Whilst I remain undecided on the optimum choice, I think that a critical factor, in either case, is to remain aware that there is no such thing as 'perfect' - remain open-minded, investigative and critical in your acceptance of knowledge. Don't, under any circumstance, allow yourself to become a clone of the person who teaches you...