Yeah his advice is spot on, you would learn a lot from the principles
You say you switch to dumbbells to lift more, maybe you should concentrate on lifting with better mind muscle connection for smooth clean reps regardless of weight used. What ever tool gets you that is what you should use.
hit your abs directly, if you strengthen your core and build some muscle it will transfer over to your strength work and also have the added benefit of having visible abs at higher body fat.
Treat your legs like what they are... pretty much around 50% of your muscularity!
I trained and stalled for a long whiled. When I dialed in the volume and concentrated on the key principle of progression things got moving with strength and physique
Eat more vegetables like broccoli, asparagus, kale, spinach and so on with every meal, pretty much as much as you can physically eat in a day. And look at some healthy fat sources like avocado, nuts, olive oil
Your calories are pretty low for someone looking to build size and strength, it may be what's holding you back. It seems you may be worried about getting a bit soft but with a higher calorie clean diet and some conditioning work you can keep that in check whilst being in a surplus and building some mass.