I always tend to fall to "Occam's razor" when thinking about such constraints on a project between material resources (usually cost vs length thing 80% of the time) and what a client wants vs can afford.

So...in this case, we have isle post that are beyond the scope of the selected sawyer. That means either:

Chainsaw mill these posts from longer bolts (logs) yourself...

or

Employ a long structural splicing joint (aka scarf) that is well and properly "glued and clamped" with an accepted/approved structural adhesive like "PL Premium" or Epoxy. Employ the correct joint like:

kai-no-kuchitsugi (貝の口継) (Shell Mouth Joint or "clam shaped splice") This joint (kainokuchitsugi) is commonly used historically in Japan for major columns/posts without advises, and is probably the better choice of the two offered.

isukatsugi (いすか継) ( halved rabbeted (oblique) scarf joint or "stork's bill scarf") if employed for a post must be in one of the "housed forms," of the joint like, sumikiri isukatsugi (隅切いすか継.)

My 2¢ at this point...

PLEASE NOTE THAT THE "FUNNY NUMBERS" ARE THE KANJI CODE FOR THE NAMES IN JAPANESE IF ANYONE CARES TO DO A PROPER GOOGLE SEARCH...JUST CUT AND PASTE...