Your thinking is about right. Having built two rakes, one for 3 point years ago and one for a garden tractor, and being on the verge of building a new one for the 3point(using the tines I bought for the garden tractor), a few design hints and comprimises come to mind. If I may make a few suggestions:
1. Having the tines not pivot is ok, just be aware that it won't corner very well like this and will spill collected debris out the side when you turn sharp as the tines will be drug sideways. It is best at collecting debris in straight lines. That is how my last one worked the best and I found myself planning straight passes to collect debris into piles that I could more easilly pick up later. If your land is anything like mine, you will want to drive around under the trees to pick up sticks and such, and doing this in a straight line is difficult. The tines farther back and with a taller frame, will allow more debris to be built up in front of the tines. The one I built for the garden tractor works so well, it can occasuonally collect so much matrial that it stops the 27HP sears GT5000 that I own. If the frame is too low, accumulated debris will actually lift the rake tines off the ground and start to spill material out the back. Unfortunatly a longer arm between tractor and tines will increase the difficulties in turning. There is no real elegant way to have it follow thru turns unless you attach it to the drawbar and allow it to pivot there and tow it like a trailer. If you allow it to freely pivot any farther back, a tine hooking on one side on a rock, stump, whatever, will cause the rake to pivot unpredictably. If you tow it like a trailer, then connecting it to the
3PH lift to raise the rake and drop off collected debris is more difficult. Being able to angle the tine rack and lock it there can be handy for windrowing rocks and debris.
2. I would seriously consider adding gauge wheels behind the rake. You can make them out of wheelbarrow wheels and a little bit of steel for probably less than $50. This will help keep the tines in constant ground engagement without digging too deep, particularly if your terrain is uneven. This will help keep you from making more debris to pick up, such as dislodging rocks that are now just below the surface. Neither of my previous builds had gauge wheels, the next one will.
My chain harrow is also pretty good at picking up small debris as long as there is not too much of it, and it follows easilly around modest curves(attached to drawbar with pull chains). But as the debris builds up, it clumps up and this lift's the harrow off the ground and the accumulated debris rolls out the back. It does consolidate the material into these clumps which are much easier to collect than the individual pieces. You might consider getting one for use after you pick up the larger stuff with the rake.
Good Luck and show us some pics of your build.