You know, what I found for me is that being IFR proficient also has much to do with how proficient I am with the system I have (mine is the GTN 650 xi). Those times early in my IFR flying when I wasn't as sharp was because I was distracted too much by not working the GNT like the back of my hand, working the GTN too slowly.
I's suggest that spending an hour, heck even a half hour before each SP practice approach flight with the GTN simulator is necessary. And work in your ForeFlight, I Fly, etc. as well since you'll be using those for the plates. Make a game out of it - run through your planned flight, and then toss in things like mentioned above.
"Hold at this fix", "proceed direct to XYZ fix", etc.
Load the full approach, and know how to quickly bring it up to go direct to a fix.
What is activate a leg vs direct to?
What if you go missed, go to the hold, and then are cleared? How do you then reactivate the approach?
What information does the GTN give you about altitudes at each fix?
You're planning on runway 01, but when you get to the fix the wind changes, and you need ATC to get you to 19 approach instead. How do you manage your I Pad to get the plate as well as change the GTN?
I'd go so far as to say a ground school session, game show style with students would be helpful. My IFR training? No instruction on the GTN. That was a miss IMHO.