I do not think it is possible to adjust the DHCP options when access point is created with ipv4.method=shared, Possibly through setting 802-11-wireless.mtu in the AP connection profile will help you adjust the MTU.
Alternatively instead of using shared, you may start your own instance of dnsmasq in a privileged container with host networking mode and set corresponding iptables rules for Internet connection sharing in a similar way NetworkManager does that.