Natural wood preservatives for swing set

I'm getting ready to build a swing set for my kids and I'm making it sturdy enough for grown ups too (because why should kids get all the fun?). I would like it to last a long, long time so I can keep playing on it after my kids grow up but wood starts to break down pretty quickly in our rainy weather, particularly where the wood touches the ground.

Going with the wood that is readily available around here, we are going to use Douglas fir in 4x6 10' beams for the top and 4x4 legs.

What kind of natural wood preservative or preservation methods could I use on a swing set? I've seen products that you soak the bottom few feet in and the usual stain-type treatments. I found this list and I'm curious what you all think of any of these products/methods:

Linseed Oil
Tung Oil
Pine Tar (this sounds too sticky for a playset)
TimberPro UV Internal Wood Stabilizer (they list playsets as one of the applications)
Eco Wood Treatment (a kind of stain)
LifeTime Wood Treatment (another kind of stain)
Charring it (and would that weaken the wood at all?)

I used the Timber Pro wood stabilizer for the parts of my chicken coop and run that are in ground contact. I have been super happy with it. It was very easy to apply and seems to be working great. I used it on the coop floor and that still looks new. The boards in ground contact, including posts all look and feel solid. I was worried it would be smelly during application and curing, since I'm really sensitive, but it was barely noticeable at all. It hasn't been an issue for the chickens either. I have to admit, I also just think it's cool that it basically turns the outer layers of the wood into silica crystals. I looked at oil finishes when I was researching options. I'm sure they're great, but I really liked that this was a one time application that was easy to do.

I love that you're making the play set grown up friendly. Swinging and hanging upside down are quite therapeutic.