The 1 day waiting period is actually a very efficient measure against brute forcing. With a randomly chosen 6 digit PIN, an attacker will on average need 0.5 * 106 attempts to brute force the PIN. Ignoring the ramp-up phase, this corresponds to (0.5 * 106)/365 = ~ 1369 years. If you wish, you can increase this time by a factor of ten or hundred by simply adding another one or two randomly chosen digits to your PIN. Under the assumption that the throttling works as intended, this is a meaningful protection for almost all users.
That said, the implementation of a duress password feature is in the works: https://github.com/GrapheneOS/os-issue-tracker/issues/29