If your racing you need to get stronger. There are reasons why 2x* makes sense.
If you want to go back to 3x* do so, but be aware that if you pick the right chainrings and cassette there is very little benefit to be gained. and you do gain a few problems.
Seems like you have more problems with 1x or 2x with the rate the rear mech wears out.
As for climb size, Le Chable to Col de Gentiannes in the Val de Bagnes on a 2×9, in prep for the Grand Raid Cristalp. Is that a big enough climb?
Really depends on the gradient, I’m not familiar with that climb.
Climb hardness has nothing to do with size, the south downs vs the Cairngorms, they are different nothing more.
Well South Downs are rolling and mellow so no probs on a 2x or 1x, haven’t ridden Cairngorms but I imagine the climbs are steeper and longer. I know you can have a hard steep climb thats reasonably short, and these kind of climbs one after another are when the granny ring comes in handy.