There’s no conditioner you can put on that will prevent scratching. You know this…. you have skin and all kinds of moisturizers! None of 'em is armor.
Perhaps the best, non-obvious advice I can add is to be careful about over-doing it with one kind of conditioner or fatty/oily/glycerine-y stuff. Too much on fine-grained leather will make it look nice “just sitting there”-- you’ll be tempted to create that nice soft glow every time with some kind of product put on with a damp sponge. But be sure you aren’t creating those “grease jockeys”-- little pills or slivers of dirt mixed with your conditioning product. Look especially at where your stirrup leathers hand or just above where your calf makes the most contact with the flap.
That’s a sign of too much conditioning for this fragile, fine-grained leather. And taking grease jockeys off requires friction or a chemical like diluted ammonia or soap wet enough to act as a detergent. All leather hates this (just like your skin does), but fine grained leather doesn’t withstand it well.
You are better off doing a little less with this leather, day-to-day. That’s especially true if you are compulsive about cleaning tack and you have a “symptom” of that addiction like sticky bridles. To me, cleaning really well and oiling and then conditioning as needed is the only way to go for fine-grained calf-covered tack.