Collar ties are generally for the benefit of the rafters though and your rafters sound more than big enough.
Removing roof collar ties.
If i could just remove that i could get to probably 75 of the loft without any acrobatics.
There is 3 supports on the loft which i believe are either called rafter or collar ties which divide the loft into quarters.
2x4 collars yes i use a piece of 2x4 placed immediately under the existing ridge like a collar tie but all the way to the top on each pair of rafters.
You might actually need 2x8 rafters in such a case.
So the top of the roof is in compression and collar ties near the upper roof should also be in compression not in tension.
Collar ties are the appendix of roof framing an evolutionary dead end in home building hanging on by just a couple of nail points.
An exception occurs however even near the ridge if the bottom of the roof triangle is not secured against stretching or flattening by a ceiling joist or a low placed rafter tie.
The only time collar ties do any good is when they are specifically designed to be functional by an engineer or architect and are installed with meticulous care by the framer according to detailed drawings.
Well unless your roof is concrete tile and less than 3 12 in slope.
Watch this video before.