I guess the other thing is How Grrrr, you are going to be, are you likely spin along admiring the view or do you have set in stone daily mileage goals? If you r taking it easy the 36/11 should be ample…
It’s what £20-ish for a triple front mech, maybe another £20-ish for a 3spd LH shifter, plus a £10-15 each for some Shimano Rings, (say 32/44 maybe), so call it ~£70 to get you set up with a wider touring gear range, are you certain your rear mech won’t cope? Call it another £25 for a Deore or similar and your laughing, I’m sure some of the bits you need could be rustled up for way less than half of that if your nice to people (I have a couple of triple front mechs and possibly some old rings if you want, your welcome to them if they’ll fit)…
You only need a cheap robust temporary drive train for the trip, the SLX can all go back on when you get home…