Welcome to the board. Trying different milk is probably the right thing to do. Weak curds are usually a result of poor milk. However, you mentioned how different recipes call for different amounts of rennet. The thing is, rennet comes in a wide range of strengths, and for some reason recipes often leave out the strength information. For example, I have 3 different rennets, for an 11 litre make I use 0.6 ml of the strongest one of them, 1.6 ml of the medium, and around 6.5 ml of the weakest! So, just over a factor of 10 between the extremes. The thing to do is to first start with the amount recommended on your package of rennet. When you add your rennett to the milk start a timer and float a small yogurt container (cleaned and sanitised) in the milk. If you tap it it will sail around freely. Every minute or so, give it a nudge and observe what it is doing. It will eventually start to move less, as the milk thickens and starts to set. Start tapping every 30 seconds, or so, until you find the point where a nudge won't move the container anymore; it snaps back into place like a cartoon character with their feet in glue. That is the flocculation point. You want that to happen between 10 and 15 minutes after you added your rennet. If it takes more than 15 minutes, use a bit more rennet next time, if it sets up faster than 10 minutes, use a bit less. Each make adjust until you get the floc point in the 10-15 minute range. (note, you may find that different cheeses will require slightly different amounts of rennet since the temperature and acidity of the milk all influence how the rennet works).
So, let's say you've got it to 12 minutes. Now, you will see in some makes that people say "I used a 3x multiplier", which just means, take your 12 minutes to reach floc, multiply by 3 for 36 minutes, and cut the curds 36 minutes after adding the rennet (note, you've used up 12 of those 36 minutes already just getting to the floc point). What this is doing is adjusting the procedure based upon how the milk is responding to the current cultures and rennet - so you will get more consistent outcomes by doing this rather than just following a set time that most recipes list.
Hope that is clear?