Not sure if anyone has mentioned it because I haven't read every reply. However, as with many challenges we encounter in our efforts on our homesteads, farms, homes, etc., most have already been solved hundreds of years ago. Timber and log framing has been in practice thousands of years. If I were wanting to keep posts surviving for several decades, I'd look to how the Japanese and Chinese have solved these problems. Japanese timber framing is astounding. Here's one relevant thread in Permies you might want to read: https://permies.com/t/40487/scribing-posts-stone
I would also reach out to Jay C. Whitecloud to discuss your specific application.
I've reached the age that I find it's much better to put my self away for a few moments and look to others who have successfully solved whatever problem I'm facing and adopt what they did. Then, I can bring myself back into the picture in the execution of the idea. Way too many people who are way smarter than I am have done nearly everything already. It's silly for me to try to re-invent the wheel.