I use a KS47 suppressed for pig hunting and was thinking of upgrading.  It hasn't been reliable with good ammo so I don't want to try subs with it.  I don't use subsonic ammo but would like to start.  8.6 ammo is similar in price to 7.62x39 subs.  I would have to buy everything including a new suppressor.  My KS47 has 8 inch barrel and brace.  I would like the ar10 to be similar or 12 inch barrel.

Is anyone making .338 bullets that will reliably expand @ 1000 fps, or less?  Bullets at least that don't cost $2 or more a pop?

I’m currently building a 8.6 SBR

AP upper/lower
Faxon BCG / 12” slim barrel / handguard
Radian supressor friendly charging handle
Q supressor
AP small parts with Magpul furniture
Optic - tbd

One piece at a time helps keep the $$ pain somewhat under control

Only about half there…

Maker makes them, about $42 per 50 and expand down to at least 900 fps.  I use these in my .338 Spectre and .338 GRRR.  I shoot a 165 gr. bullet up to 2200 fps but have tested them down to 900 MEASURED fps with full expansion.  Maker also makes heavier bullets, 300 gr., maybe more, as well as bullets specifically for the 8.6.

I take it that you do not reload so a wildcat, or semi-wildcat, is out of the question?  Sub-sonic is sub-sonic, smaller rifle, lighter weight, same oomph on the receiving end with less recoil.
.338 Spectre
complete upper
And they use a standard 9mm can so you don't need a can rated for the .338 Lapua or anything like that.

Absolute cheapest way is probably a complete upper from Bear Creek, they're just under $400.  I have one of theirs in 762x39 and it isn't pretty, but it runs well.  Then find the cheapest lower you can.  Brownells had Aero M5 lowers on sale for $120 over the weekend.

While BCA is cheap, the time you spend doing your own QC/Tweaks to make it serviceable might not be worth it. It depends how much you value your time. Also if anything needs warranty work (New Barrel Arriving as a Sewer Pipe) OP will be out his upper/barrel until its sorted out.

Faxon makes a 12" barrel for about $200. I had the .458 Gunner Version and it was pretty nice. Though in hindsight I wish I purchased a Tromix for that caliber.

Faxon makes some good stuff, Those Gunner profile barrels balance really well.

https://www.joeboboutfitters.com/Faxon-Firearms-AR-10-12-BIG-Gunner-8-6-BLK-Barrel-p/fax-10a863p12ngq.htm