When to start timer on pre-infusion machines for recipe target times and how to make sure my grinder isn't slipping?

TL:DR - when do you start your timer on a machine that auto preinfuses and how can I best calibrate/check my grocery quality isn't slipping (it's 5.5 years old and ground probably 200 lbs conservatively but there burrs look good).

This is a wall and I apologise in advance.

So here's where I am mentally, I've got twins on the way so I figured I'm going to need quick caffeine and have always wanted an espresso machine in general. I wanted a good entry level and already own a 2012 Baratza Preciso. I'm honestly not sure what burs I have but I am pretty sure that I have 2.0 gear box.

So for the money and features I bought a Breville Infuser at bed bath (by the way they did take a 20% coupon. You just have to find a manager that will do it, so call around if you're having issues). I figured that the Infuser gets good reviews now that this update gets hot enough, and in terms of longevity, I've never had any issue with BBB replacing something even if it's been ages so if it goes down I will have them replace it.

Here's my question. After about half a pound of coffee I was getting decent shoots at 17g coffee: 34g espresso in 27 seconds (when I hit the button) even though the pressure guage (this annoys me that the Infuser has this anecdotal pressure guage as opposed to actual numbers. Reminds me of those shitty bbq guage that just have "the bbq zone" as opposed to numbers) was on higher end the scale.

I programmed the voumetric double on the machine too so I'm getting consistent weight out at 17g in. So m keeping everything as consistent as possible.

Then I went and bought a verve bag that I liked and everything went to shit. Within a range of 2 adjustments on my grinder 17g would either not get to pressure and dump 34g espresso in like 8 seconds or would be so highly pressured that only 6g would come out in 40 seconds.

Finally got it dialed in enough (after going through 3/4 of the freaking bag) where I was getting my ratio recipe in 42 seconds or 17 sec and after daylight savings kicking my ass and 45 minutes of dialing in I just said screw it and drank it.

So Question 1 - how can I check that my grinder isn't slipping and calibrate it correctly. I feel like once I got into range it shouldn't be so crazy to get dialed in with such wild swinging results. I feel like my dose amount, tamp are consistent, and if the pull amount of water is consistent it seems like the only variable.

Question 2 - at what point shound I be starting my timer on my pre-infusion Breville infuser for recipe target times?

Any help would be appreciated.