Do I need the spacer on the inside of the 42t cassette nearest to the flange? The internet seems to say no but without it the lockring won’t tighten the cassette enough, with it and there’s not much cassette lockring thread going into the freehub. Shimano tech docs dont mention it.

Yes, you do. 1.85mm. Wheel / hub should have come with one. It’s so the hubs can be used with 11spd road cassettes which need a wider freehub body.