I use a T15 upright threading insert in a toolholder from Warner Tools to cut metric threads on my 2nd lathe, which I keep set up with the metric change gears. Carbide threading inserts will break when you try to retract them after stopping the spindle with the footbrake; T15 isn't bothered by that if you fail to get the tool retracted just before you stop the spindle. I've done a number of Howa builds, to include truing the receiver face & lug seats. Even single-pointed the threads on a few of them, but finally decided the threads are usaully plenty good enough so there's no need to true them. It's pretty easy to make a threaded plug that can be screwed into the receiver threads after you've got the action dialed-in. You can then check how true the threads are by indicating off this plug - they'd have to be fairly crappy before I'd dive into picking up and single pointing internal metric threads again. Don't mind cutting external metrics, though I don't do it often enough to stay really sharp at it, so I mentally go through the drill a few times before starting.
The difficulty in removing OEM bbls from Howa actions used to require the parting tool cut right up against the receiver face, but since they started using the HACT 2-stage trigger, something's changed, and now I find that pulling an OEM bbl from Howa actions is generally a bit easier than breaking a Rem M700 bbl loose - this is true on both the full-size 1500 & the newer Mini actions. I routinely pull the heavy bbls to thread muzzles, and haven't run into an issue for the past couple of years.
There didn't used to be a lot of aftermarket support for the Howas, but now PT&G makes Stealth DBM (there may be other suppliers who also do the same), which is very easy to inlet for, although you do have to do some mill work on the projection from the rear of the magazine well to get clearance for AICS mags. Timney makes a decent aftermarket trigger, though the HACT is one of the best OEM triggers I've seen. Warne & EGW make extended scope rails for them, in both 0 MOA & 20 MOA sloped models. Warne also makes rails for the little Mini actions. B&C makes a new #2093 stock with clampbar adjustable cheekpiece, very similar in shape to some of the more popular tactical stocks, plus Manners & McMillan make a variety of hunting stocks for the Howas. It's a good action, with some very good features, like a one-piece forged bolt, M16-style extractor, & integral recoil lug. In my experience, their QC is consistently very good.