On the No-Name, the Bristol, the Yucatan, the Improved No-Name, the Improved Bristol,
etc. With so many knots and variations out there it pays to focus on technique and principles.
Pick one of the good knots and for me that is the Slim Beauty, learn it, practice it and go with it for the applications where it's called for. I always lube with lip balm, and then follow a couple principles. Twice through is better than once in most cases (speaking of Uni-knots and hook eyes), more wraps give insurance up to a point and, yes, using a double line increases the break stength of the knot in most cases. I've also found that in a pinch the difference between tying a Bimini to get that double line can be dispensed with and you can just double the line, or use a twice through (surgeons) standard loop.
For instance tying the blood knot to make leaders, I often double the smaller piece of leader, especially when trying to match diameters and I don't use any knot to get the doubling but rather just fold it over and go. You are going from a 20 lb test to a minimum 40 and more likely 60 lb test line so take your pick and practice. I suspect a doubled line, blood knot, aka the Stu Apte Improved Blood Knot will give you a break strength between 90-115% of the stated break strength of the 20 lb test.
Measure the 20 lb test leaders actual break strength and it's likely upwards of 25 lb test then tie your knot of choice and measure again. I always use lip balm on larger diameter lines as saliva just isn't slick enough to seat heavy material firmly. Were it I, given your conditions, I'd likely tie the slim beauty with a doubled line doing some at home in prep, but on the water go to the Stu Apte Improved Blood as it's a tad easier. In both cases I feel the strength of the knots would be close to if not over 100%.