You never mentioned they were out under the sun. The more intense the light is over them the more nutrient your plants will take down. Assumed these were indoor because pics look taken inside. This explains why half strength nutes were not doing the job. Cannot tell you exactly how much to feed them; That’s up to your own judgment to decide. I don’t even really like most bottled nutrients but obviously if upping the dose was good then staying the course seems the way to go forward.
Potting up to a larger container with compost and fertilizer is vastly more preferable imo to dumping on a higher ppm nutrient solution but you said you lacked the vertical space. You gotta do what you gotta do to keep them green...