My answer to OP may be better placed in response to Wagcat as he addresses objective and subjective approaches... We can only subjectively define The Good Life based on our limited understanding of Reality. Once we fully define Reality to a scientifically acceptable extent, then we know there is either an objective purpose or not. If there is an objective purpose, we orient ourselves appropriately. If there is not an objective purpose, we subjectively define The Good Life knowing all the pieces to the puzzle (i.e. Reality). The kicker is that we won't fully define Reality in our lifetimes, and it may never be fully defined to a scientifically acceptable extent... the purpose to life thus becomes living a subjective good life within our current understanding of Reality while constantly pursuing to better define Reality to a scientifically acceptable standard.
My original reply to OP...
"It's simple once you accept that you won't have the objective answer in your lifetime nor will those 10,000 generations from now, if even then.
Assume there is an objective Reality.
Realize you will die before humans are able to fully define that objective Reality to a scientifically acceptable extent (i.e. there are an estimated 71 billion, trillion stars in the visible universe... we've got a long way to go).
Do everything you can (e.g. learn, educate others, promote cooperation, maintain our habitat, etc.) to help our species (even 10,000 generations from now) fully define that objective Reality to a scientifically acceptable extent.
Promote, based on our most current understanding of Reality, The Good Life (which will include the pursuit of objective Reality until fully defined).
Once objective Reality is fully defined, there is either an objective purpose or there is not. If there is an objective purpose, we orient on it. If there is not an objective purpose, we fully understand the playing field (i.e. Reality) and define The Good Life appropriately.
The virtuous good life is not in the pursuit of Happiness... it's in the pursuit of Truth. Your legacy will come from how you used your life to support that pursuit for others, and your honor will come from having known you will likely never see the fruits of that pursuit."