They used to advise a higher concentration, but reduced the official recommendation to “90% or higher” (except for biocompatible, which is 96%+).
Weakening the concentration won’t improve your results. Disposing the old alcohol and replacing it with fresh stuff will.
How many washes you get really depends on how much excess resin is going into the bucket (size and surface area of part, amount of residue on it, whether the geometry has cavities where resin pooled). If you remove the parts from the build platform first, and use the wire basket, that may stretch the working life of your wash solution by a bit.
You could also consider a manual “dunk bucket” - i.e. dip your part in a “dirty”, pre-wash bucket first, even let it sit there for a few minutes, before placing into the Form Wash. That would greatly reduce the amount of excess resin contaminating your Form Wash each time. Before the automated wash station came out, some folks would maintain a “dirty” initial bucket and “clean” final bucket. When the initial bucket became too dirty they’d dispose of the contents, clean it, fill it with fresh alcohol, and that would become the new “clean” bucket with the moderately used alcohol in the other one being demoted to the “dirty” role.
Age plays a factor as well. Like you, my partially-contaminated alcohol sometimes sits unused for months at a time and gets a bit gross. Big bits do settle to the bottom and congeal, so siphoning off the top may let you stretch it a little, but I’m not sure it will be worth it. As resin dissolves the whole mixture becomes less effective.
Another thing I’ve noticed helps is keeping some pristine alcohol in your rinse bottle, and giving parts a squirt with it right after they come out of the Wash - especially any nooks and crannies. You can use an alcohol-soaked Q-Tip with a wiping motion at the same time - a bit of mechanical cleaning action is really effective at removing tackiness. Note moderate amounts of lingering tackiness will completely go away after a full cycle in the Form Cure (or other UV oven).
Finally, you might consider switching to TPM - I hear it can absorb 3x the amount of resin before needing replacement.