If you set up a chainsaw properly and use the correct ripping chain, on the correct mill, you can produce lumber that looks planed without waves or roughness. I would suggest buying or making a ProCut type carriage mill. I do not bend down, I walk up right, and 8 turns on the crank = 2", so making live slabs is very easy.
> Question: what do y'all recommend to extend the life of a chainsaw used for milling?
Do a vacuum and compression test every so often, I am pretty sure leaking crank seals is what killed my Husky 385, which was a wonderful saw for the mill. Though my 1st saw for it was a 365, followed by a 372. Personally, I think a MS-066 running a 32" bar is the ideal setup for a chainsaw mill. Soon I (Creator willing) will be using my MS-660 FarmerTec clone for a procut carriage type mill made from scrap metal :-D , mounted on a dual axle trailer, with a winch and ramps to load the logs onto the carriage.
My trailer carries all the gear I need to cut down trees, 3 chainsaws, pole saws, 250+ feet of rope, 400+ feet of 1/4" cable, 35 feet of chain, pulley blocks, come-a-longs, Peavys, steel bars for rolling logs, axes, wedges, tools, etc. PLUS, it can carry firewood away OR it can carry lumber away.
What is paramount is making sure your chain is parallel with the kerf, if so, with good ripping chain you will breeze through 24" pine and produce boards that looked planed. Plus, I can turn logs if I want to make dimensional lumber.
I had a Husq 3120 with 36" bar and ending up selling it, just nothing big enough to cut on a regular basis to keep it, especially for dropping and bucking trees. Plus, I hate not having a side chain tensioner. I used a customer's MS-880 on his carriage type sawmill, nice saw, but in the smaller stuff it is no faster then the 385 or MS-660, plus, no side tensioner. So, imho, unless you are cutting hardwood logs over 24" diameter on a regular basis, you can do without buying a MS-880 for $2000+.
If you are cutting logs under 32" and mostly pine, I would get a saw with a side tensioner, so, chain swaps are quick and easy, that runs ripping chain .050 x 3/8" and a 32" bar (popular size) which is good for 27" logs on most mills. I run Stihl 40:1 with non ethanol gas (87 octane), with octane boost, with Stabilt, and my saws and equipment have been running fine on that.
Have fun no matter what you pick for the job because what we do, many people could only wish for in their wildest dreams.