Seat post bags yet or nah rack and bag

In terms of the bike itself, I appreciate you only want one bike, but it is a lot to ask for something that is suitable for commuting, suitable for light MTB use and still light/racy/aero. A "racy" gravel bike is probably your best bet (so if not the Crux then something like Canyon Grail) - although you typically won't get the most practical features that are useful for commuting. 

There are some good seatpacks out there. Whether or not one is suitable for you is an open question. It seems to me the main drawbacks of seatpacks for commuting are capacity (typically small), shape (e.g. very few will take a laptop), and mounting (generally a lot more faff than taking a pannier off and on). Given your budget, something like a Tailfin might be an option to get full pannier capability on a bike without dedicated mounts. 

Lights - for off-road riding it's  generally recommended to have both a handlebar light and a helmet light. Exposure do a couple of sets which are presumably intended for that purpose, but I haven't tried for off-road riding. I do have Exposure lights for road riding and they are very well made, although expensive. Plenty of reviews out there of other options though. 

You want a Fairlight Strael.  All the mounts you need and stupidly fast on road or gravel path when stripped back.  Sexy as hell too if custom drawn 853 is your thing.  Will take 38c tyres with guards at 650b with a bit of fettling of Bluemels or similar bendable/cuttable mudguards.

I used to run one (2nd hand from Ebay) before going one better with a Reilly Gradient (identical geom to the Strael) but much wider tyre clearance.

Love my gradient but still sorta miss the Strael.

If you want a dirt cheap seat bag to dip your toe in the water have a look at Podsac's ones on the Planet X site.  You can get a full set of bikepacking bags for not much more than £100 - especially if you hang on for one of their discount weeks.

A Condor 'Fratello' would be fit the bill with bags from Restrap.

Trek Domane SLR is in your budget. Rack and mudguard mounts, 4 bottle mounts, endurance geo. 35mm tyres with guards I believe.
(As long as you've got somewhere secure to lock it up)
The aluminium version is similarly practical and discounted heavily at the moment. So you can spend anywhere from about £800-£4k on what's essentially the same bike

I've never been a multiple bike owner.  Both my previous bike (a Charge Juicer) and my current one (Ribble Endurance Ti) have mudguard eyelets, but not rack ones.  With both, I've used a seat post clamp with rack mounts and the mudguard mounts to fix a rack.

With that budget, I'm not sure why you would compromise, plenty of bikes out there to find one with the right configuration. What is now called a gravel bike is exactly what is going to fit your flexibility needs, but as you've found different designers have different ideas of what a gravel bike is from something akin to a tourer to a gravel race bike.

My Giant Revolt 2 was way less than your budget and has rack and guard mounts, double GRX groupset, hydraulic discs. I've got a second set of wheels for gravel and endurance. Basically winter bike and gravel bike.The only negative of riding it compared with the Defy is tyre weight as it has 32mm tyres (deliberate choice for winter road riding) which are noticeable on climbs. Unfortunately my Defy has 140 discs so I haven't been able to prove it is the tyres (without effort!) but the Revolt is a great all day rider. When I had problems with my summer bike, the Revolt was fine as a road bike, just noticeable that I had to work a bit harder up hill.

I've got Flinger guards on it. Don't try and get away with lightweight guards, they don't last and are rattly. It takes a couple of minutes to remove them or replace the Flingers once fitted to size.

Using a rack puts the weight lower down.

So my suggestion is as a first pass, stick with full guard and rack fitting. Only if you really can't find what you are after consider the workaround options, especially if commuting is going to be the main use.

The obvious answer is this

Mounted on this 

The problem is that the biggest capacity bag is made of aged obsolete materials with obsolete buckles

I bought a gravel bike, which I use for commuting. It doesn't have mounts for a rear rack so I'm using a seat post bag. It works OK, but it's not ideal. I find the shape is the main issue.

I'm going to need to carry a laptop on my bike from next year, so I'm getting an Ortlieb Quick Rack. There are a few similar racks for bikes without mounts - Tailfin looks very good, but it's also very expensive.