My household has 3 sets of servo wave Shimano that I work on, and we've been on those solely for the last 3 years. SLX 2 piston, Saint, and a pair of SLX that I converted from 2 piston to 4 with new Deore calipers. Every time I've experienced wandering bite point (WBP), it's been fixed by burping air bubbles out of the lever.
Whether bleeding the whole system or just doing the lever, it takes a LONG TIME to make sure all the air is out of the lever. Tap the lever till the bubbles stop coming out the bleed pot. Wait a minute, do it again. Wait 15 minutes, do it again. Do it till you can't possibly spend any more time on it, and WBP is gone for a very long time. If it comes back in 4-5 months, just burp the lever with the same process again, and you're good for another good span of time. A vaccume bleed system would likely be the answer for quicker/more consistent bleed results.