This answer might be unfavorable and considered crass by some but...
The first red flag is For a few years, the founder was unhappy about the technical skills of the employees
Have the employees attempted to rectify the unhappiness?
The second red flag is two of the five developers never used SQL before
It's hard to create an efficient system if the developers are not familiar with core technologies and truly understand what the ORM is masking.
It's hard to imagine that I worked for twenty-five years in this industry, and you? What have you done? You've been a code monkey for three years. So shut up, you, moron! Nobody cares about your opinion, a******. was actually uttered but I will take it at face value and believe you.
However, do consider the first red flag I mentioned and the "unhappiness" the founder has had to deal with for years.
To this, I add: you guys have known about the founder's unhappiness for years?!
How was this information divulged to you?
I am inclined to think that this guy is doing precisely what he was hired to do; get you guys into shape.
Getting you into shape does not refer to adopting the new guy's bad practices but it does involve throwing you out of your comfort zone to learn at a deeper level.