I've done both a couple times and will just submit that an engine swap is much easier than an R and R of the head, in terms of special tools and paying attention to fine details. Time involved is the same. Engine requires more attention to safety but otherwise you take it out, you put the other one ...

Probably cracked head or just head gasket

You can’t move ECM units in Volvo’s after 1998

I'd have a machine shop check the head, it might just need a quick shave.

Volvo horns awarded for removing 6 thou of warp with sandpaper!

That’s dedication !

Did you use new head bolts? That’s recommended by Volvo but I’ve always wondered if they are truly in the deformation limit

I had a head resurfaced at home as well. Was 0 mm W and 0.05 mm L in one spot only. Managed to have all zeroes. Finding a flat area of a 3/8" thick glass. You get the head as flat as the glass is. https://a.d-cd.net/uQAAAgG17-A-960.jpg Dykem steel blue (wasn't really needed) https://a.d-cd.net/...

No pressure! The head is heavy enough, you just need to push and pull it, as horizontally as you can. Because of a fine grit and wet sanding it took a few hundreds of passes, but in return it gives you a precise control of how much material is removed. I stopped when I had a couple of cavities reduc...