I'm curious, is the time spent mixing rennet into the milk very critical? I have seen recipes that say "mix for no less than 5 minutes" and others that say "mix for two minutes clockwise then two counter-clockwise" and some advise to mix for as little as 30 seconds. I understand it's important to mix it in thoroughly but can it be over mixed? This may sound petty but I get the feeling it's an important step.
You want to mix it enough to be thoroughly incorporated and no more. Rennet when it hits the milk starts cleaving the outer shell of the casein micelles. When that happens, the micelles join each other and start forming the curd matrix. If you keep stirring, you are disrupting that formation, which may result in an extreme case in shattered curds or wear curd set when you start to cut.
Recommended motion is top-down do ensure all of the rennet is distributed throughout the milk, including the milk on the bottom of the pot.
TIME
I'd recommend 1-2 minutes, I set a timer so I don't over do it. I stir quite vigorously and use 1 minute. I once stirred for 4 minutes and found that the curd was already starting to form and I was breaking it up! Over 5 minutes is way way too long.
METHOD
Up and down is the preferable method and without breaking surface (no splashing) to minimize introducing oxygen into the milk.
STIRRING TOOL
I think this is less critical but I use a skimmer (https://cheeseforum.org/forum/index.php/topic,2073.msg15393.html#msg15393) in my stockpot which I've noticed when near surface has a really nice efficient mixing effect of milk going up down through the holes. When I used to make smaller 1-2 US gallon batches I used a whisk in horizontal motion but believe a skimmer in vertical is better.
Thanks for the great advice! I am going to try another batch today, wish me luck :D