I've never dealt with old aged sawdust so I'm guessing a bit here: Sawdust is a strong brown and if fresh would deplete soil nitrogen. At 10 yrs. it may be pretty well broken down. If it's still N-hungry and your plants doo poorly in spring, you could add high N fertilizer to correct it. If you have time you could do a test now, mix up a small batch and plant some seeds (a pot would work) and see how they do. If they sprout and grow OK it's fine.

I did have to lime, soil was about 4.2 and manure was 5.6. I did that well before mixing in the manure to have time for the soil pH to adjust.

FWIW, I've planted in really well decomposed "ground tree" (so included some leaves and composted by itself) from utility work, that was probably 10 yrs old. Don't know about the sawdust. Again, test samples from several places in the pile (mixed together) before using it as a soil amendment.

Sounds like the manure was still working - what did it smell like when you dug into it/dumped it?

I hope that 150 yard figure was a typo, if not that's 15 dump truck loads of soil to work with. Yikes.

You want in the end a soil with 5-10% organic matter, but when adding compost initially you can put in a lot more than that, for two reasons: A significant fraction of compost is minerals and water; and, as the compost continues to decompose it gives off some of its organic matter as water and CO2. So you could add compost up to as high as 1/2 the volume of soil. Just remember it's going to shrink down, so pile it high in your beds.

I'd still get the manure tested (if you think it's done composting - sounds like it might not be), then you'll know how much N is still in it, you don't really want to mix it 50/50 with the sand, but you need to know what the composition of that "topsoil" (which probably is mostly/all subsoil) is before you start amending. Mixing straw in sounds good, esp. if it's not quite "done" yet, that should help keep the process going.

You probably won't need to add any "fertilizer" at all, just dig more manure in each year (planting legumes in rotation will help too), with as much as you've got available. But you really need to know what you're starting with for soil.

If you could get someone with a skid-steer to dig out the rough bed down two feet. It would then be easy, easier for two people, to set the ties till they are the right height for you.
You could use screw or spike to hold them together if desired, then leaving one end open, fill by driving the skid-steer in and tamping as it fill, so your soil is not so loose and will not settle. Then close the open end and use the skid-steer to fill remian gap and the gap outside the box.

This way you would not have nutrient bleeding off to the sides but would be contained in the box going straight down if they do move.
At the same time your base would not be rocky sand but the same soil you grow in.

Just an idea.

Check with your county extension office about soil testing - ours does it for free.