I have a small boat that will be launched later this spring and I will use either the stick or anchor and rope method.
I like using a 10' piece of 3/4" PVC pipe. You should be able to easily feel when you tag the bottom and you can mark with a Sharpie.
Then try and push the pipe INTO the bottom. For an old pond with lots of organic "muck" you should be able to push through significantly deeper. Hopefully, you will then be able to feel the "original" bottom of your pond. That might give you some useful information for future pond management if you ever get the opportunity to clean out or deepen your pond.
Did you ever put in your ten extra largemouth from your stocking plan? Hopefully the answer is yes. However, I would still consider bucket stocking some 10-14" bass as esshup suggests. (He is a pond management professional.) I think you need the larger size to eat the larger brooder Green Sunfish. Further, some of the bucket stocked bass will probably not thrive in your pond. So not all of the number that you add will become effective GSF predators.
(When I first joined Pond Boss, I thought moving bass from one nice home to another nice home would still result in healthy and happy bass. However, the pros have pointed out numerous situations with tagged fish that some just do not respond well to the environment of their new home.)
With a large population of Green Sunfish, I think you need a period with too many bass! You will need to make a large dent in the GSF population to get your pond in balance.
The good thing is that LMB are one of the easiest fish to remove at a later date. If the pendulum swings too far in that direction, it will be much easier to identify that you have stunted bass and need to start culling some, compared to the very difficult job of analyzing your GSF population.
Have fun and don't stress about making an "error". Any slight miscalculations performing the tasks you have mentioned above could not cause an "unfixable" error in your pond - in my non-expert opinion. (Letting your drain pipe stay clogged and then having a big rain washout your dam is what I would consider a true error.)
Good luck improving your pond!
