Lot of stuff in here... I'll try to address it in order...
-If you set your time to zero and press and hold the "manual" button, you can pre infuse as long as you like, (until you see first drops) and when you release the button, the pump goes to full. Just thought I'd throw that out there because not everybody knows it
-Re your pump cutting off during pre infusion, I've not had that per se, but on my six year old -900 I had it where it would be reluctant to pre infuse, then eventually it would not pre infuse at all. Replacing the pump, which I did myself, did not fix it. So I sent it off to Breville. They ended up sending me a new machine. In my case, I think the problem was on the control board. But I think lots of low power pre infusion is hard on the pump, so a pump change might still fix it. According the the Aussie repair men on another forum, changing the pump usually fixes it. Fortunately, it's pretty easy and the pumps are not terribly expensive.
-RE resisting modifications: The Slayer mod (which actually gives you more capability than Slayer, is fully reversible. It's almost not like a mod. All you are doing is re routing existing tubing, plug and play.
-By the way, pre infusing until you see first drops, then running the pump full, then tailing off the flow rate towards the end of the the shot is a classic Slayer shot profile. It tends to improve light roasts quite a lot. If you are happy doing it this way, no need to mod.
-More thoughts: one of the key "ingredients" of "Slayering" is very low flow during pre infusion. On the order of 1-2ml/s. You can measure this by pre infusing into a cup on a scale for 30s, the weight you get in grams is also your volume in ml. One of the reasons for the Slayer mod is you can get it really low. But again, unless you are playing hard with super light roasts, you are probably fine not doing it. Still if you do the mod, you lose nothing in the way of extraction capability. You can still use the machine like stock by opening up the needle (water knob) and using the machine as you always had, before the mod.