This calculation is incorrect. It's not as simple as just adding 88 bits to 12 bits to reach 100 bits. The majority of seeds which could be brute-forced from knowing 16 out of 24 words won't be valid because the checksum will fail, and so there is no need for an attacker to then start brute-forcing diceware words for them - they can simply be discarded. It is more accurate to work out the length of time to brute-force 83 bits to calculate all possible seeds, and then the length of time to brute-force 12.9 bits for each seed.

Your estimation of 1 quintillion guesses per second is obviously way out of the current scope of computing power, but if you are looking decades down the line to potential inheritance, then who knows what will be possible. For illustration, reducing the entropy in the calculation from 100 bits to 83 bits, then your number of 40,000 years instead becomes around 4 months.

Shamir's Secret Sharing as mentioned by hatshepsut93 is another option, but be aware that there is no standard implementation of it. If you do use Ian Coleman's site do split your seed this way, you should be keeping a couple of copies of the code in various places in case his site and GitHub disappear from the internet in the future.