I just need to convince myself that I can make good splices and protect them sufficiently.
WW,
You don't have to be an expert to do this kind of waterproof splicing.
There are thousands of people in the US who use submersible well pumps. I have replaced 5 or 6 of these pumps for family and friends over the years. These pumps fail for many reasons - pump sucks in mud or gravel, casing corrodes and leaks, lightning runs down the wires, motor burns out, and so on. The pumps come with a stainless steel sealed motor housing, and 2 wires coming out of a sealed fitting. At first, when a pump would fail and someone would ask for help getting the old one out of the well, and a new one installed, I always looked at the splice between the factory wiring and the user wiring (which is completed submerged under 30 to 40 feet of water for years and years). I never have seen a pump fail because the waterproof splice failed.
Here is the way these are made. They take a heavy-duty clear plactic tube (sleeve) about 4 inches long, and 1/2" diameter (depending on wire size), and install rubber grommets (doughnuts) on the ends. These rubber doughnuts are not to keep out water, but serve two other functions. They help center the wire inside the tube, and hold in the epoxy filling until it hardens. You insert one wire into the tube, push it out the other end, make the splice using the supplied crimp terminal, then pull the splice back into the center of the plastic tube. Then you squeeze the epoxy into the tube, completely filling all the space until all the air is out of the tube, and there are no voids, and the spliced wire is centered in the tube. Then you let it harden for 5 or 10 minutes. I would bet money that the wire insulation will fail before this type of splice leaks.
A handy person could make their own splices like this - I am sure that is how they first were invented. All you need is a good heavy-wall transparent tube, some 2-part epoxy glue, and maybe some rubber or cork stoppers for the ends. You could just tape the ends up with electrician's tape. The epoxy is the seal, all else is for convenience to get the epoxy in place.