Consider the following:
Rent/buy/borrow some regular boat jack stands. I have no idea what the right
number is, but a visit to any boat yard will give you more information than
I can. Just look at all of the boats on stands for work or storage.

LOWER the front of your trailer with the trailer jack. Your stern will now
be higher than normal. Place two or more jack stands at the stern. RAISE the
front of your trailer with the trailer jack...place additional jack stands
where needed. When you have the boat standing on all jack stands, the
trailer will be free. Then go back and by using additional jack stands,
raise and lower the front of the boat, inserting the "extra" jack stands
where needed; while gradually removing the trailer from under the boat.

You might also get away with very large blocks of foam, instead of jack
stands. I've seen a number of boats put up for winter with just blocks of
foam.

Warning. I have NOT personally done this, but have read it as a solution to
your problem a number of times. Do a Google search and see what you get,
and I'll bet a number of solutions will be the above process. RichG