If I see a hill that I know will require me to use the granny gear, I shift to it in advance and start on the smaller cogs in the back (maybe 3rd smallest out of 10 in my case - I have a 11-23 cassette in the back).
Shifting on the granny when RPMs are already low (in the 60s for me) takes away too much momentum, as you experienced... Plus, the low ratios of the middle gear are equivalent to the highest ratios on the granny gear so you're not compromising much... (Hum not sure this is clear .....

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And if I want to stand up on the pedals I upshift two cogs. I try to NEVER go under 8 km/h at which point it's too late for anything to happen anymore!!!