You seem to have been scared away from mid drive chairs - and it's true, some of them are junk. But it sounds like you're likely going to get a none custom built chair - somewhat off the shelf. With that said your also likely to get a RWD chair where your too far forward - if you 100% want RWD, try and find something that can keep you weight back. You see the thing with the Mid chairs is, the drive wheels have good traction - all your weight is on them. They feel a bit worrying because they're more like unicycle. LOL what? Maybe a bad analogy. But heres the thing, the other 4 wheels are like stabilisers. That's all they do. They articulate to allow the chair to handle uneven terrain and just stop it tipping. The fast ones often go around corners on 5 wheels. The inside front will lift in the air as the rear outside squats into the turn. They typically offer really good traction.
However... You might just not like how they feel. Or how they feel a bit 'wobbly'. The best 'more extreme' ones are larger which causes issues indoors. The 4 corners are usually wide and long, regardless that they turn on the spot, they have a lot of swing - a corner is going to wack something in a tight spot. SOME are much more compact, but then also less extreme.
Check this out if the v6 is to bighttps://www.magicmobility.com.au/wheelchairs/ the 73 v6 is shorter, the v4 might be your thing... but you could do with testing some chairs if you have any big dealers near you. They're expensive but if your talking 15 years and not wanting to go custom build - I can't help but think you're talking about your environment, and your experience... and they sound like your chair is not good. But you want another one.
Your right about FWD, the industry is obsessed with FWD. It's mostly your RWD in reverse, but their typically longer and the rear casters get in the way more than RWD front casters do AND you can't see them. IE RWD you look where you go, you point, you shoot, your done. FWD you always have to account for the rear swing, hitting a wall, table, chair you can't see it, your always judging it. The other thing is FWD when your going up a ramp, you might have to go up it backwards especially if it's wet or slippy. The front wheels are trying to pull you up the ramp and they can't typically because there's now too much weight at the rear!
Your toy hauler is really cool, but I don't consider that ramp to be steep at all. I would happily go down that forward unaided in any RWD chair I've owned.
IF invacare are they only chair you can easily get a hold of you might want to try and get a test on a TDX.
It looks like you have a MPJ+ Joystick - fyi they're not gears, they're just different profiles that change the parameters of the joystick, for example profile 1 might have a max speed of 30%, but you turn the speed pot down and it'll run at 10% but the max is 30, and profile 3 might have a max of 95% and the turtle makes it 70%. profile 4 won't give you any more traction than any other setting, but it will help you apply the power more gently which might spinning the tires by being to heavy on the power. Personally Every chair I have is on max, always.
As for beaches you might be surprised what we can get away with because our weight is a big advantage. If you can get your weight moved a bit better - the front casters plowing in is a big issue. the rear just tried to drive them into the ground. On sand it helps to have a feel of the chair, as soon as the front starts biting in you can usually turn out of it before it's too late.
Here's some beach action with some basic RWD's.... If you get a chair setup A BIT better, you can get away with a lot more.
Below you can see my tracks in the sand, I've come from the grass hill straight ahead with a basic RWD chair
I have air filled on my chairs with some OXO tyre sealant in them and rarely get punctures these days. Also when you buy tyres make sure you get 4-ply ones as these are stronger and less likely to puncture. 
. I can see this chair being detached and then tucked into the back of Mom's "Journey" SUV as well.
and it don't use the inaccessible LINX too. 
