The proceeding is what I think is going on, and could be wrong.
Think of it like this:
Rapid Synthesis - lowest durability hit, lowest quality, highest progress
Standard Synthesis - medium in everything, perhaps slightly higher chance to succeed
Bold Synthesis - highest durability hit, highest quality, lowest progress
If it's something you are having trouble making reliably with standard, do rapid every turn. When it succeeds you will get a massive boost to progress, sometimes close to 30%. However, don't do rapid if your craft is sputtering and sparkling. It has a very high chance to fail at this stage and the durability penalty for failing a rapid synth at this point is enormous. Try to wait it out.
For Guildleves, the reward is directly tied to the quality of your finished synths. Firstly, try to finish all three or however many you get. Then, try doing rapid up to about 75%, and if your durability is still pretty high, do some standard to get your quality up. If it's something you can do pretty reliably with standard, try it.
Bold Synthesis is for recipes that are far under your skill level. It has a low chance to succeed on synths that are even to your skill and lower, and murders your durability whether or not you blow it up. Succeeding on a bold synth when your item is in either stage of instability gives you tons of quality, but like rapid, if you blow it up (you're going to blow it up) be ready to take an even bigger durability hit.