I'm not going to be much help in soil. All of my soil stuff is outside and I basically just use my homemade compost.
I would suggest later, if you do what I recommended as far as your new seedlings, try one in coco vs soil. If you go to my journal ask me any questions and I can answer if you like the results I get.
Meter - You can keep throwing money at it I guess. You might look into a cheap Lux meter and do the conversion to PPFD? Just concentrate on PPFD for now. I think there's something by Dr. Meter that's around $30? Maybe someone who has used one can chime in on that. I've heard it's pretty decent. I wouldn't run out and buy a $1000 phone. And not sure how good an iPhone 7 sensor is. Maybe it's great.
That said, I'm running close to 1000 PPFD right now on my plants, and they are just eating it up. No issues. But maybe you did burn your plants? Is it just the tops?
I'm not trying to discourage you either man. I want you to do the very best that you can. But sometimes you gotta slow down a little and take in the sights. Learn to observe your plants and let them show you what is up.
This isn't a toxic place like Reddit. Lots of knowledgable people here who love helping. Look through the indoor grow journals by some of us. If you see results that you would like to get, ask about them. If you ask me a question in my journal, I'll give you more info than you wanted!
Here's a link to the lux meter, but you have to do the conversion manually I believe:
Amazon product