> The point of breaking things down like this is to minimize reviewer context.
This principle is much more important than some rule that says "Merges to main should not be more than 150 lines long". Sticklers for hard-and-fast rules usually haven't achieved the experience to know that adhering to fundamental principles will occasionally direct you to break the rules.
> Merges to main should not be more than 150 lines long
This can be done by allowing a flag in the commit message that bypasses the 150 line long (or whatever example) rule in the CI that enforces it. Then the reviewers and submitter can agree whether or not it makes sense to bypass the rule for this specific case.
In many cases like this, it's okay to override a rule if the people in charge of keeping the codebase healthy agree it's a special case.