So... where can a non-PT find this kind of advice? Maybe you, or any other trainer you can recommend? Or some online resource?
(Personally, I'm probably above-average knowledgeable about fitness, but I still find the information unmanageably overwhelming - train every day, every 3 days, every muscle once a week, every muscle 3 times a week, big muscles less often than small muscles, focus on strength, focus on growth, focus on flexibility, cardio, no cardio, ...)
You probably need to be more specific about your end goal. Is it to lift a particular amount, run a marathon, climb a certain route etc? There are usually specific trainers for certain skills, just like in video games :) If I wanted to get better at bouldering, I'd book a 1 on 1 at my climbing wall, not a generic trainer at the local gym (but maybe I would after advice from the climbing coach).
For generic "get fit" goals, stretch every day and some amount of cardio/resistance training three times a week is hard to get wrong. The important thing is to read your body and don't push your luck. Also, to stop when you lose good form.
There is a lot of overwhelming information, but a lot of it is conflicting and anecdotal. You yourself point out - should you go for growth? 5 days on? 7 days on?? You can find information online to support pretty much any theory you like (particularly about training frequency).
I agree there's a lot of conflicting information and that specificity is key. But it can also be useful to have a guide for when you change goals, someone who knows your history and preferences etc.
Body composition changes typically go through multiple different phases and cycles. It might seem like you'd want someone specific for each phase, yet some consistency is helpful across phases too. As an analogy, you wouldn't want to change universities for each year of your degree
I was trying to allude to this a bit, but maybe it didn't come across clearly. I was thinking more about how you approach getting help to achieve a goal. And probably a good way forward is a mixture of occasional 1-on-1 sessions with experts in your chosen activity, followed up with more regular sessions with a general trainer who can help you achieve what the expert says you need to improve. For example, specialist advice can range from very specific technique improvements, to more general comments like "technique is good, but you need better core strength" at which point a personal trainer might be a good call.
I think that's essentially what online training is these days: a cheaper way to get access to a fitness 'sherpa' to guide you. Someone who lives and breathes this stuff to save you a lot of time and effort. Most of the real, practical, and current knowledge right now is spread across a variety of $1000 courses for personal trainers.
I'm in the process of building the largest online fitness database of its kind to democratize a lot of this information. I'm not sure what sort of business model I will pursue yet. But I want to collaborate with likeminded people and make the foundational information open to the public.
(Personally, I'm probably above-average knowledgeable about fitness, but I still find the information unmanageably overwhelming - train every day, every 3 days, every muscle once a week, every muscle 3 times a week, big muscles less often than small muscles, focus on strength, focus on growth, focus on flexibility, cardio, no cardio, ...)