I am not the paragon of all wisdom, but this is how I shift uphill on my 1x: As I crest the tall hill, I can feel my cadence (pedal speed) increasing and will start shifting to stronger gears. My bike is an old mtb, retrofitted and with thumb friction (non-indexed) shifter, so I can do several cogs at one time if necessary. I don't know if you can on your modern setup. Nevertheless, when you go to shift, just reduce pressure (or ease up) to make the shift. As others have stated, under pressure is where things clunk and jam a little. So, waiting to crest is usually a good idea. Scanning ahead and "anticipation shifting" is something else I do. When I see a hill coming up, I will shift to keep cadence same to a little higher. It's amazing how fast momentum dies on the uphills
