Can I change the default sysctl values in grub?

The feature you are looking for is available only since kernel 5.8, as described in this UL SE Q/A: How to set sysctl using kernel command line parameter?:

sysctl.*= [KNL]

      Set a sysctl parameter, right before loading the init
      process, as if the value was written to the respective
      /proc/sys/... file. Both '.' and '/' are recognized as
      separators. Unrecognized parameters and invalid values
      are reported in the kernel log. Sysctls registered
      later by a loaded module cannot be set this way.
      Example: sysctl.vm.swappiness=40

So combined with devconf_inherit_init_net found by OP in a previous Q/A, starting from kernel 5.8, but not before and thus not before 5.1 either, adding through GRUB these two kernel parameters:

sysctl.net.core.devconf_inherit_init_net=1 sysctl.net.ipv6.conf.all.forwarding=1

will achieve the intended goal.