Never edit lk with a hex editor. The bootloaders on GitHub have mostly all been DMCA'd by Mediatek, and the bootloader for Astro is on a private repo. No plans to release yet.
There is one I was working on, but I'm not now as I want some clarity from PC about the DMCA.
You can avoid the unlock message by using an officially signed image, and having the bootloader locked. However, this makes rooting impossible.
The way we avoided this in Gemini and Cosmo was using the bootloader as signed, with 'untrusted' boot slots - BOOT_3/BOOT_4/BOOT_2/RECOVERY_BOOT_2. This has not been implemented for Astro yet, so I'm afraid you're stuck with the delay for now.