With 2.75, going to Canada, I guess its tight.
Probably doable, but very, very very tight.
Thought I saw you mention 19.45, but at 17.45, maybe/probably/possibly ok.
Its probably doable, if you are on the same PNR, I have never been to the US/Canada, so can't say anything about extra security checks, if they do still selective extra security processing at the gate.
For SYD D - I, I would leave 2 hours, max.
For me, I only travelled Au - NZ, and its minimum procedural checs, Au - NZ.
I have had to do it in 1.75 one time, SYD - WLG/SYD - CHC, can't remember now, this was pre covid, pre rat test/pcr testing, I know they don't need/probably don't need to show evidence of vaccination now.
I had to run from the QF bus drop off, up the escalators, to the immi and security processing, was in J, but didn't have time for the lounge.
Got to the gate as boarding line was already very long.
This was pre covid, ADL - SYD - WLG/CHC, on a same PNR, ie, through tix.
Is yours a same carrier, ie, QF all the way?
Truth be told, I am not sure what would happen if you miss the connection, but with 2.75 hrs, its probably ok.
If on different carrier, I would be worried, if its all on QF, its probably ok.