Max amperage for “neutral tie strap” in daisy chained Siemens panel
The bonding strap will be fine for as long as the breaker on the live side feeding the current needs to trip (unless otherwise specified in the documentation of the panel).
To connect higher gauge wires that are too large for the neutral bar holes you can use a neutral lug which is a little metal block that screws into one of the neutral bar screw holes and provides a place to land the larger wire. And yes it's better to put that block on the same bar that the neutral lands on.
Also when installing a subpanel always go for more spaces than you think you will need. The small premium you are paying now will be much less than the cost of yet another subpanel in the future. As long as the wires feeding the subpanel are appropriately fused the rating of the main breaker in the subpanel doesn't matter.