Thimbles are useful, but they are best used with a slice rather than a knot.
Knots.... they can reduce the breaking strength of line/rope, or not, depending on the knot used. Bowlines run in the 90-95% range as I recall, they are easy to tie AND untie. Another useful knot for the purpose at hand is a tent hitch. Adjustable and quite strong, easy to untie.
https://www.netknots.com/download_file/533/0
