If apple doesn't go to them, I think the only other option is Samsung? But I don't know that Samsung's fabs are open to third parties.
Samsung has produced parts for others for a Very Long Time, most famously NVIDIA (not "nVidia" since many decades) use(d) them.
Shorter-term that's true.
Apple is well known at times to think longer term on logistics, strategy. If push comes to shove, there are two additional options for Apple.
1) Political. Taiwan not being China depends on the support of the US. Apple is a economic security prize for the US, a strategic asset. Apple can get powerful politicians in the US to lean on Taiwan, which can dictate TSMC's behavior to an extent. This would prompt increased compromise from TSMC (3-4% hike, not 6%).
2) Apple starts investing into fabs and builds a TSMC competitor from the ground up, perhaps in alignment with ASML (Europe very much wants an improved semiconductor context) and a few US partners (build the fabs in the US and Europe for national security purposes). Apple is one of the few companies where you have to take this potential seriously, they can safely bankroll it entirely by themselves if necessary.
They most definitely are, my employer has sent trial tape outs to them recently.
Apple also has something TSMC desperately needs too: the ability to pay upfront and guaranteed volume.
Turning these lines on and off is basically impossible so guaranteed volume is a way to smooth out demand and is needed for planning.
These factories are tremendously expensive and getting cash upfront is basically a zero interest loan in a time of high interest rates.