Lets get this straight.
First you completely bypass the safety features of your local power grid by using an isolation transformer.
Then you attach a grounded strap on your wrist. OK, it probably has a resistance built into it to limit the current, but still, IT IS GROUNDED.
Then you are puzzled when you can feel a shock. Granted, a small shock, but still a shock. That resistance in the ESD strap is limiting the current to that small value.
My first thought is
YOU DO NOT UNDERSTAND ELECTRICITY! I spent over 45 years working professionally with all kinds of electronic and electrical equipment. Voltage levels from 5V or less to 40,000V or even more. I worked on high power transmitters and on flea power logic circuits and virtually everything between.
I NEVER WORE ANY KIND OR ESD OR GROUND STRAPS. NEVER!
Why not? Because I valued MY LIFE more than some static sensitive IC which cost mere pennies. If you or I touch a point that is hot, that ground (ESD) strap COMPLETES the circuit and current can then flow.
But I worked with those static sensitive ICs and other components. In 45+ years how many did I destroy with static electricity discharges?
ZERO!That's not an exaggeration. It really was zero devices destroyed with ESD. The need for protection against ESD is tremendously over stated. It is probably only needed in the warehouse where the ICs and other static sensitive devices are handled by people who have no idea of how things work. They are not working with energized circuits so the ESD protection devices are a lot safer to use there.
Likewise, for isolation transformers. Again, in 45+ years of professional electronic work, I NEVER used them. Actually, looking back, I think only ONE SHOP that I ever worked in even had one. And these were professional shops usually with a number of very experienced technicians working in them. If you have an isolation transformer between the mains power and the device you are working on, you have DEFEATED any safety measures that your local power company and local electric code has WISELY incorporated in the grid up to the outlet your transformer is plugged into. And, if you bypass that transformer with a ground wire, then you may as well not have the transformer in the first place.
Somewhere outside of your house or shop, your local power company has, GUESS WHAT. A TRANSFORMER! And it is configured in a manner that was determined by years, decades actually, of experience with electrical safety. You ALREADY have a transformer! Adding a second one is redundant and unnecessary!
If you really want additional protection against electric shocks, I would consider installing a ground fault device either in your breaker box or at the outlet you are using in your shop. But that may drive you nuts with multiple trips if any of your equipment has a leakage path in it.
DO learn how to work safely with electricity! But, in doing that DO understand how it actually works. PS: I also never liked the ESD mats for the work benches. If I was stuck with one, I DISCONNECTED the ground strap before it allowed me to get shocked.