Missing one, or perhaps an alternative to the hybrid … cargo bike!
My currently being updated list is:
Road bike to which mudguards can be fitted so it can be all year round bike.
Gravel bike, again with full mudguard mounts for all year riding.
Trail Hardtail MTB with mudhugger guards for winter/mud.
Full sus XC for summer/big MTB rides.
E-Cargo bike for round town.
In reality, winter MTB has been overtaken by winter gravel riding, so actively debating ditching the trail hardtail.
Going back to the OP’s question, it would seem there is insufficient distinction between the riding done on the MTB vs the riding done on the gravel bike. So to me it would make sense to ditch the hardtail and be done with it, or ditch the hardtail and replace with a more capable MTB so you have the option of doing trails for which the gravel bike would be way to far out of its depth.
If it’s the amount of time you spend on each of your bikes that bothers you, riding more might be another way to go. My old GT Force road bike - which I’m very attached to but have hardly ridden - has moved from the garage to the home office and is now my indoor bike. I don’t have any more time because of this, but it’s made it more likely that I’ll spend that time riding a bike (kind of).
I think if you get some use out of all of them each year and have the space (and can store them inside so no maintenance necessary if they go unused) then there’s no need to n-1 it. Gravel and XC bikes can occupy a lot of the same space though so if one is much more fun to ride then you may find yourself never riding the other.
I’m on 4 at the moment:
Road, actually my old gravel bike but essentially ‘all-road’ now, I used it for road and gravel for 4 years but wanted something with bigger tyres off road
Gravel, 50mm clearance and with mudguard mounts
27.5" hardtail, goes unused because it’s too burly for the XC trails where I live but not enough for the bikepark/mountains. The main n-1 candidate
Commuting ebike
I have all the parts bar the frameset and wheels for another bike so have been debating building n=5 winter bike up at some point. But in the spirit of this thread maybe the current set up, another set of wheels for the gravel bike for winter, and an XC/downcountry bike instead of the hardtail would work best. Then I could build an n+1 bike my brother with all the parts I have spare.
I say keep them all, plus do the upgrades to the Crux you were thinking about. Even if you don’t ride that often anymore…or perhaps especially…it’s good to have exactly the bike you need for those relatively rare rides. Hence the variety. Make each ride as perfect as it could be, with the perfect tool for the job.
On the other hand if you are constrained by budget/space and have to get rid of some bikes I would agree the Crux is the most versatile. Great for gravel, and decent for road. Not so great for mountain bike rides and maybe too nice for a townie. Agree with a spare set of wheels - one for road and one for gravel. As for gearing, consider something like a 42 or 40 tooth chainring and a 10-52 eagle rear cassette. And switch to ETap of course. And a dropper post. I have a Framework “all-road” bike with this setup and it’s good for road as well as “light” gravel. You don’t want to be switching drivetrains and this gives you a very wide range. For my more specialized gravel bikes I use the XPLR drivetrain with the 10-36 cassette and 36 or 38 tooth chainring since I want to bias the drivetrain towards the lower end. My Litespeed Ultimate G2 - which is setup for winter with dynamo, fenders - has a 36 front ring and 10-52 12 speed Eagle. Need those extra low gears running studded tires in snow/ice/slush! Bikes I use more for road or touring/bikepacking I tend to go with a 2x setup. I have a Rodriguez Phinney Ridge set up for touring with a SRAM 2x drivetrain. 10-36 rear and White Industries crank with either 48/32 rings or 46/28 depending on what the route will be. I only point this out to suggest that you don’t need to be limited to the SRAM crank/chainring options if you are upgrading. Plenty of other crank & chainring manufacturers (Praxis, White Industries, SRM, Lightning, Easton, Rotor, Cybrei, Garbaruk, etc…). Good luck!
Another option? Sell everything and get one premium bike somewhere along the all-road or gravel spectrum. Depending on what you think your primary use is, you can decide on the nuances of the geometry and tire clearance you’re looking for. Something like my Ultimate G2 can be set up so many ways, Lynsky has similar, or Dean, or many others… Or more upscale or downscale depending on budget and preferences.
BTW, I forgot Framework Bicycles is now called Forma…apparently there was some conflict with Frameworks Bicycles which I think was started around the same time. Framework/Forma is in Canada while Frameworks is US-based.