I'm still thinking a high end XC hard tail would be great - assuming you are OK with flat bars and can get the gearing you need. Race training on my 17lb gravel bike when everyone else is on XC hard tails, I'm faster than the group (which is all stronger than me, lol) because I have comparitively light wheels/tires so I can climb faster, and with better aero and gearing I can blow them away on a slight down hill (or anything powered over 30mph). So - weight, Aero, gearing. But, If I'm not sprinting, and the bikes are comparable weight - my speed is pretty similar on a gravel bike or XC hardtail.
(IMHO, modern MTB handlebars are way too wide for the road).
Of course, active suspension is the latest thing in Gravel bikes (although that can come with a weight penalty). For you, I would rent a XC hard tail for the day from your local shop (most of mine are happy to arrange something - especially if it is a potential sale), and do a good long gravel ride to see how the thing feels to you.
What size tires/wheels are you riding on your Raleigh Roker Comp?
"somewhat on a stiffer ride side than a gravel bike" It's very different on a mountain bike compared to a drop bar gravel bike. To me "stiffer" isn't even part of the equation. Like @chas suggested, i'd rent a mtn bike first to see how you like it. I find them much less aero than a drop bar and the hand positioning is also very different. Considering the distance, this could be monumental for you. Much more comfortable for me on drop bars.
I dunno... you may be overthinking the need for a front suspension mtb for unPAved. Yes, it would help in that short section... but overall? i've viewed a couple of videos and from what i can see all are riding rigid. Perhaps you need bigger volume tires, tubeless, lower pressure, and maybe, a stem shock, or Lauf fork on a drop bar gravel bike at the most. Or just more time in the saddle with bigger tires to up your skill in those rockier conditions. You can always drop your pressure when needed, then pump up again.
I'm still learning, and have never done unPAved, so take what i say with a grain of salt. I do know going from 47c tires to 2.1 vastly increased my own confidence and ability in rockier terrain. I wouldn't even consider my "retirement bike" (last?) until i thoroughly knew the differences personally between a drop bar gravel bike and a flat bar mountain bike.
The Otso Warakin Ti would be a great choice, I just bought the stainless version and love the ride, responsiveness and versatility of the bike. The Tuning Chip is a very well thought out feature, and the bike is nicely built, with no corners cut and lots of configurable options.
Another option I haven't seen mentioned yet in the thread (I could have missed it!) is the T-Lab X3, which is made in Montreal and uses some very innovative forming techniques to optimize horizontal and BB stiffness while maintaining vertical compliance. https://t-lab-bikes.com/Bikes/All_Terrain/x3.php ... it's at the high end of your range depending on the build you choose but still within.
Searching is half the fun, and has the side benefit that you will KNOW you have found the right bike for you.
I'm going to throw my two cents in. I'm 46 and was just pondering the other day what my retirement forever bike would be. Currently I am riding a Cannondale Scalpel 2019 Hi-Mod (with a Fox 34, not Lefty). The typical riding I do is this: local grassroots XC/Shorttrack Mountain bike races, middle pack age group. 3-5 hour mixed surface/pavement gravel rides - this usually includes rolling out from my house, riding 1.5 hours on street/pave bike path/gravel to get a mountain bike trail, ride a mountain bike trail for 1.5 hours, and then return trip home back on via paved street/gravel/paved bike path. The occasional gravel race between 50. This bike is perfect for this type of riding for a couple of reasons: 1. Two water bottles in the triangle. This can't be understated; 2. Full lockout for both the fork and shock; 3. For washboard gravel I unlock either the fork or shock but not usually both - this really smooth's things out. The older I get I think this would be more important for me.
I really can't ever see myself, health permitting, not riding some form of singletrack yet the older I get I'm enjoying more and more big miles away from traffic on dirt roads where I can just put my mind in a zen state and just sit on a low tempo/endurance pace all day. For me a xc mountain bike fits this bill. It's not a perfect bike by any means. After about 60 miles of mix use (gravel rides) you really start to feel the weight of the bike on 2.25 or 2.1 width xc tires. I'd say 60 miles is about the limit I'm interested in doing on a flat bar mountain bike. Anything approaching 100 miles and I would lean towards a drop bar bike. In some ways I'm the same predicament as you - there isn't really a bike that does everything well. I contacted my LBS and a Cannondale Topstone Carbon just arrived and I'm picking it up this week and going to test ride that. The idea is that if any rides are going to be 60'ish miles or more then I would grab my drop bar bike, anything under (which would probably include dirt and single track) then a XC full squish bike is the go to.
Short take away: if you are routinely planning on doing 60+ mile mixed surface rides then a drop bar bike. Anything under 60 miles I think an XC mountain bike is a no brainer. It's just to comfy and suspension platforms are soo efficient today there really ins't a reason not to. Let's face it. As we age, comfort is going to become more and more important. Thanks for listening.
