I will say that some of your characterizations are accurate, and others are not. I've not worked with Peter before so I won't comment on him specifically, but your assertions about law vs. clerical work is spot on. I will add my own input as well.
Immigration attorneys in the US are similar to hacking groups, minus the illegality. Good hackers have their own 0day exploits. Good attorneys have their own techniques for obtaining legal status in the US. Dealing with USCIS/ICE/EOIR/State is akin to bureaucratic hacking. Anticipating what the Government agencies want to see and hear in your petition is their exploit. Finding the appropriate supplementary evidence is their payload.
I don't think your assertion of solo attys vs medium sized firms is accurate. In my experience, solo immigration attorneys focus on the easy to win cases (like K-1 visas and AOS). This segment of the market is diminishing because of the fact that it _is_ mostly clerical work.
Likewise, the big firms are simply too big and expensive to want to approach one-off cases. They deal with the FAANGs and Fortune 500 companies.
For those reading this, consider that everyone has exceptionally complex circumstances that surround their immigration status and legal options here in the US. Obviously Peter cannot comment on it and OP has not shared that much information.