I suggested rEFInd in your other thread as hopefully an easy workaround. I’ll assume it didn’t work for you, or you didn’t like it for security or other reasons.
I don’t have a lot of experience or knowledge to help you specifically with what you’re asking about, but I’ll point out that it’s a similar setup to what some people around here are doing. You can search for keywords like detached boot partition and find things such as this: Install Qubes OS with boot partition and a detached LUKS header on USB although you can perhaps adapt and simplify this for your case if you don’t need to also detach the encryption header.