So you "just don't like the general feel". Over the long haul soaking up a lot of the potholes, ruts and rough roads will not pay off. That comfort is sucking up power and adds weight to the bicycle. Have fun and you may flat out overpower some riders but it is not going to be a competitive bike.
Gravel racers and bike manufactures have come to what is considered an optimal design and they all seem to be in pretty close agreement.
It is near the end of CX season - you can often find some great deals on used CX.
The line between CX and gravel is pretty narrow. Since CX is a 1 hour race they tend to be stiffer. But I own a very comfortable TI CX.
Running larger tires at a lower pressure will absorb a bit (but maybe not optimal performance). Not all CX / Gravel take up to 40MM (and some take even larger).
DK200 recommends 40mm+ tires. Even on your mtn bike you should not ride narrow tires.
It is not the ruts and pots holes that wear you out but gravel size vibration. A good carbon fiber fork with a large soft tire deals with gravel sized anomalies in an optimal way.
As for shoulders/neck discomfort drop bars have multiple positions to spread the fatigue. I just don't see how aero bars on a mtn bike are the way to go.
At 200 miles you are into offroad endurance racing. Look into off-road endurance bikes like a CutThroat.
That is a pretty brutal race. Over 1/2 the field is going to be out right animals - you would need to be a near world class mtn biker to be competitive on a full suspension bike.
OP comments on new bike selection. We don't really do recommendations here but I suggest you break it down by what you want and translate that to specs.
If you are serious and have some coin then you will need mud and hard pack set ups. Buy an extra set of wheels and experiment with some tires. If you have a support team then you dould even change out at the check points.
What dominates the needs is the 40+mm tires. It addresses both race course and comfort. Not all gravel bikes are going to take bigger ties. That is why you need to look into endurance off road. I would run tubeless and run them at the mid to low end of the pressure. In that size it will actually be mtn bike tires. If 40+mm really limits your options then look at at smaller. The down side to off road adventure is that in a shorter normal gravel it will be a little slower than a pure gravel bike. But you have a bike you could off road backpack. Of even put touring tires on it and use it as a touring or commuter. It is kind of hard to $2K - $3K for just one race - are the other things that could help justify the expense. Look used - there was used Fargo in my town for just $1200 recently.
When I compare your modified bike to what I would consider an optimal setup it is not close. Not even sure you could add lockout to those shocks. If it takes replacing the shocks then very costly. Add up what it would cost to optimize your bike compared to a new optimal bike. From there make a decision. I can tell you drop bars may seem clumsy at first off road but after you get used to them you will get it.