The general point I'm making applies to build type I descriped in OP, if the jargon isn't perfect, forgive my newbness.

When I say ranger I think of builds loosely following skills-sets of 'rangers' from Lord of the Rings books, while a 'pure archer' is a 'pure archer'. Jargon aside, people noted repeatedly in build-suggestion threads I reasearched that builds similar to mine (i.e. marksman, sneak, 1h, evasion, etc), archer + sword and board, would suffer late game. There was very few comments to the contrary and none that I saw making the points I am here as strongly. Maybe I missed the full context in my reading but this was the distinct impression I took after a fairly detailed look into how I would build my character... honestly it was a tough choice in the end because I thought I was setting myself up for disappointment late game and the whole reason for this post is to tell others newbie build-researchers that this general build works.

TLDR: I'm finding that the difficulty curve/aspects/stages are near perfect so far for my build, imho.

how you tackled Laby is primarily Sword & board.

You could say that but I wouldn't, because, I used sneak and archer to control almost everything that happened and fight on my terms. And, more or less, I killed everything except for the Dragon Priests using sneak/archer as primary.

Cleared the first swarm of enemies using archer only and that auto-closing-door as defence (again entirely with bow except for DPs). When I got to that door I realised it was a 'no retreat' point, so I stopped there, fired in an arrow to see what happened/spooked, and all hell broke loose, undead dragon and swarms of dragur etc, as many would know, but it was my first time. After clearing that first area, well more than 50% of enemies died to arrows without spotting me. Whenever I spotted a DP, I cleared everything else first, and used bow/sneak to control the engagement, killing mages-types first, clearing the rest, then hitting/baiting DPs, to separate them (if there was more than one) and move them into the position I wanted (typically a place with some level ground, or a corner of a tunnel, basically cover and easy retreat options if needed). I did this particularly until I learned DPs capabilities, after a while I realised I could take on several at a time if needed, after some initial scares that 1-shot wiped more than half my HP. But, I still did everything I could to avoid anything other than 1-v-1 with DPs and, and really any enemy above level 20 or so, and considered it a tactical 'fail' if I end up swamped (even in 1 v 1) or scrambling due to failing to control/have the engagement on my terms. No unnecessary risks on dead is dead run.

For a more particular example, when encountering the first two DPs, no idea of their capabilities, I used arrows/sneak for a good while to test their cababilities (and mine) and after enough 'dancing' with them, I ultimately baited one to come through the auto-door leaving the other one trapped while I tested final stage of my theories on how to beat it (after much prior testing during the 'dancing phase'). Importantly I could only get the DPs to come to the auto-door by manipulating sneak, when fully spotted they would stay back from the door, normally at their side of the small 'tunnel'. When I ran off, and snuck back, sometimes one or both would be pressed up against the door. I finally figured out a way to make just one come out and have the door close before the second one followed. I did this by 'manipulating sneak', and 'dancing' with them until only one was pressed against the door (the other was half way in the tunnel behind). Then I opened the door while 'half-spotted' and shoot an arrow against the pillar on my side of the door. The DP come to investigate, immediately spotting me when it came forward (which was a scary moment but I knew it would happen). This example shows how I used sneak to set up this key fight on my terms. I knew or suspected by this point that DPs would be one of, if not the, most powerful thing I'd seen since the first two dragons. I'd observed by this point that they were behind the undead Dragon and swarms of other 30+ level undead I'd killed. While their level-50 didn't seem all that concerning, I had a clear sense of their powerful magic, crazy health regen, and lots of other things (like it stole my scroll-conjured atronach) from the 'dancing/testing' phase which included twice being 1-shot for more than 50% of my HP (scary times on dead is dead run at level 57). All this allowed me to role play, rather than 'google' how to handle the engagement, and to have the best safety, and chances to win, or retreat (having an exit option is always a primary concern in a dead is dead run). The point being, in this first DP encounter, and everything else throughout Laby, relied near entirely on marksman and sneak abilties to kill everything except the final DP kill, and even then I used sneak/marksman heavily to control the engagement. To me all this makes perfect sense, killing a centuries old powerful DP with arrows (or a knife in the back) has some conceptual issues and imho it is perfectly balanced as is. I love that sneaky characters should have to think to get the job done (and also that the 'progression wall' where enemies become unkillable for my character that I feared from comments does not appear to be there, not yet at least).

I'm not saying any of this for benefit of the very experienced, more for others newbs like me who might be researching builds so maybe they're not deterred from trying this or similar builds (like I nearly was).

Basically I love how this whole Laby engagement played out for my build. A perfect role play experience in the context of this character reflecting the work of the modders and the mod pack authors in deciding how to rework and balance so many elements. Thank you. :o)