You still have 180° of angle you can use, doesn’t have to be just out at the sides.
With enough give you can have them position any way you like and you also don’t need to be just centre of the bed being off centre can change the dynamic even with a solid headboard.
Quite envious of the learning opportunities you and your partner have going forward.
Might have to find a way to have reason to buy/build a new bed.
Ah, well, for every bit of furniture we’ve bought for about 8 years, we’ve thought about sex potential. Restraints pushed us the way of wooden pillars. Very good choice too.
As @JGood said, get creative. You have the things connecting the headboard to the bed to start with.
We have both under the bed restraints (not used at the mo as we have a 9year old) and this spreader bar
My wife loves the both but prefers the spreader bar as combined with other restraints leave her feeling vulnerable and open to pleasure. As stated you can be more flexible with postions with the spreader bar without too much trouble untying and reattaching restraints.
We have these sets to compliment the bar
https://www.lovehoney.co.uk/bondage/handcuffs-restraints/wrist-ankle-restraints/p/purple-reins-body-harness-with-wrist-and-thigh-restraint/a40388g73805.html
https://www.lovehoney.co.uk/bondage/handcuffs-restraints/wrist-ankle-restraints/p/purple-reins-thigh-wrist-and-ankle-restraint/a40389g73806.html
https://www.lovehoney.co.uk/bondage/handcuffs-restraints/wrist-ankle-restraints/p/purple-reins-wrist-to-waist-belt-restraint/a36062g66476.html
For reference my wife is a size 18 and finds all the restraints comfortable to use.
Have fun whatever you try
Both are really fun, but offer two completely different experiences!
With restraints… you are restrained, not much movement and you can’t turn over etc. so it can sometimes be a bit of a pickle when you’re trying to get to certain openings! 
Spreader bars keep your legs spread, but you can flip the person over and use front or back without any issue, it also gives you a nice handle to hold when your on top of your partner missionary style 
I’m not sure there is much to add beyond the helpful comments so far…
Loving the idea of attaching the spreader bar to the under bed restraints - will be giving that a go!
We have the under bed restraints from the LH advent calendar, really useful and permanently in place should they be needed!
If your spreader bar is adjustable, they can be fun used parallel to the body - wrists at one end, ankles at the other. A very strict almost hogtie.
I took my under mattress restraints off when I got my new bed (obvs), and I never put them back on. Really must do that.
I prefer under-bed. Spreader bars still offer a little too much vertical movement than I’d like. I prefer the full body restraint.