Personally I have quite a bit of soldering experience. My first soldering iron was one of those big Weller's. I think it was 60/100 watts. Too big for sure but it got me started playing with electronics. That was back around 1969 or 1970.
I personally have seen a few million boards of various types, and diagnosed their problems. Seriously, I worked in the test engineering lab where (starting in 1981) we manufactured 30,000 boards per week (the original IBM PC) for the 5 years I was there. We were a contract company, i.e. we manufactured for lots of companies, Compaq, HP, Apollo, Adaptec, etc. Everything from power supplies to memory boards, modems, printer boards, etc. Over 5 years time, literally millions of boards. Bad solder joints are the number one problem, we had QC people whose ONLY job was to inspect the soldering. Even people who have done it for years as a living make mistakes. Just trying to impart my experience (since it is very relevant, and very extensive). I've run test labs for military avionics, NASA, Motorola, etc. Last 20 years I've been in software engineering. But soldering hasn't changed much in the past 100 years or so.
It is important to use a high quality solder. I am using Kester 44. It's my understanding removal of flux isn't necessary unless soldering on high impedance PCB's. We are just soldering a tiny tinned wire to a ring terminal or busbar. I don't see any problems with that.
Kester 44 is good, it's the "gold standard" and of course has rosin core, and is NOT lead free. Weller is great, but to solder onto a busbar, you are going to need the correct tool, a 20 watt iron designed and sized for a PCB won't work, you need something larger (like for copper pipes). Certainly soldering guns aren't expensive, but they also might cause problems with 22 gauge wire like BMS sense leads. Flux is NOT a good conductor, impedance and resistance are two different things. You are wanting to sense voltage in the millivolt range, not run high frequency signals through it or carry large currents. A good quality crimp will have just as low a resistance, has less of a learning curve, and stands a much better chance of success for people without extensive experience. Be sure to prep the busbar with a dab of flux (DO NOT use anything but rosin based flux for electrical connections), heat to temperature, add solder, then introduce the (already tinned) wire to the solder pool. Be sure to clean off the flux when done. Or, buy a crimp tool (you'll need one anyway) and the correct crimp connectors and wire strippers (you'll need them anyway as well). If your ohm meter can't tell the difference between a crimp connection and a soldered connection, why bother with solder (again, soldering does take experience). One or two crimp connections on some test wire, and you are good to go. Soldering to bus bars on the other hand is like learning on one of the most difficult solder joints possible. Speaking from experience, for an untrained person, crimping has a much higher success rate. We actually analyzed numbers and defects.
Always be sure and ohm out the connection, end to end. Someone scoffed and said it wasn't necessary on a soldered connector, decidedly an incorrect assumption. Great way to blow out your BMS.
If one is inexperienced soldering and wants to learn, then a little practice will do. Just have to make sure the solder flows and is bright and shiny when cooled off.
Seriously, this is like picking an Indy race car to learn to drive.
Someone (maybe you) was talking about solder ring terminals, that's a different story. Much easier, just make sure to clean the flux off completely. Then again, you won't get better performance from that or more accurate measurements, it'll just be the same as a good crimp.
Since this is a DIY solar forum, I'm speaking to people with little to no experience. A crimp connection is every bit as good, and much more likely to be done correctly if you are inexperienced. I understand the desire to solder, I really do, but the end results when done properly are identical, and one just has a much higher rate of success.
Obviously, it's your system and you are more than welcome to construct it as you wish. For the average DIY, crimping is a much better option and works just as well. The topic however is best practice for connecting your sense leads. The most likely to succeed is crimping using the correct tools, I'm not talking a pair of pliers and a steak knife.