Rich Idle AFR reading

I agree
When you bring in that much advance, then to keep the idle speed down, you have to close the throttle. This reduces the fuel flow from the transfers, and so you have to richen up the idle mixture screws in compensation.
Then when you slowly tip in the throttle, she goes momentarily lean until the transfers come back on line, and then she goes rich, cuz the mixture screws are set fat. To cover the resulting hesitation, you mess with the accelerator pump, making it better for a moment, but then worse.
The cure for all this is to synchronize the transfer ports to the mixture screws and let the idle-timing be what it will be, to get the rpm you want it to be or close. But after the T-ports are synced, don't mess with the speed-screw any more. If you want to change the idle speed, you do it with idle timing.
As to the power timing, it should fall between 32 and 36, once the Rs are up to maybe 3000 to 3500. The rate of advance can be tricky with a manual trans, but with a stock cam 9.2 compression engine, I suspect all-in by 2500 is too fast.
If you look on Mattax's kindly supplied graph, you see curves with kinks in them . I like those for manual transmissions. Altho I like the kink a little later and the all-in a little sooner.... than those shown, but it really depends on the combo. Hiway gears and heavy cars could be more like the ones in the graph. But a lightweight-A and 3.91s, could have a later kink, and a shorter run to all-in.
If you get it right, you should be able to run cheap gas on full timing of about 35* +/- 1*.......WITHOUT detonation.
I am no fan of setting the timing to obtain the highest rpm or vacuum, with a carburetor, AND A MANUAL TRANS. Almost invariably, this leads to driveability issues. Set the T-port sync, and things will start to fall into place.

Burbling on decel is likely a two part issue;
first is a lotta gas in the headers, and second is oxygen also getting in there. This continues the burn in the pipes, and you hear it as burbling. Advancing the timing was a bandaid, that started the fire early,to try and finish it early, so less raw gas made it into the header, from the heavy draw on the too-far-open mixture screws, and too-far closed throttles.
The cure is to reset the mixture screws to factory spec,(maybe 1.5turns out) syncing the transfers,which will reduce the closed-throttle decel-vacuum, and sealing the headers;both ends.

The idle circuit HAS to get fuel from the transfers. The mixture screws should be thought of as a trimmer.

You can set the transfer slot exposure to something like .060. Then set the idle speed by setting the timing to something like 8*;whatever it takes to obtain about 700 rpm idle.. Then trim the idle quality with the mixture screws. You might find the best screw setting to be 3/4 to 1 turn. OR
Or you can reduce the exposure to say .050, add 2* timing, and reset the mixtures to 1.5 to 2 turns.... OR
Or you can reduce the exposure to .040, add 2 more* timing, and reset the screws to 2.5.
The engine may idle equally well on any of these settings..... because it is getting enough idle fuel.
But it will like just one of them, when you tip the throttle in. It will tell you when the exposure is too little, by hesitating. This shows up on the AFR, as a lean dip, then rich.
It will tell you when it is too rich, by going mushy. Find the two points and split her up the middle, OR
Pop the carb off, flip it upside down, find the T-ports and adjust the curb-idle speed screw to make the exposure slot square. Then bolt it back on and do not touch the speed screw again. Use idle timing to adjust the idle speed. Then trim the idle quality with the mixture screws.Finally synchronize the Accelerator pump, and lean it back to factory specs. If the mixture screws come in, in the center of their range, then you are really close to perfect. Make a note of their adjustment setting, AND the rpm.
Now, using the fast idle cam, set the rpm to around 2000, exact speed not important. Make sure your Vcan is hooked up, and working. Figure out if the mixture is rich or lean. I do this by bringing a bunched up shop rag over the airhorn... then adjust the mixture screws to obtain the highest rpm, with the mixture screw setting within the limits of 1T to 3Turns. If it's not within those limits, then the low speed circuit may need work. But if it falls in that range, Then
Release the fast idle cam, and allow the engine a minute to cool. Now reset the curb-idle screw up to 1/2 turn in either direction to readjust the idle speed close to it's former speed.
Now go for a low-rpm driveability test; from idle to about 2400, and up into second gear. Use the gas pedal, tipping it in gently, looking for the hesitation.
With a stick car, this hesitation is extremely annoying; you'll have no trouble finding it.
If this is successful,in having no hesitation, record all your settings.
Then check your power timing, and rework your Distributor to get the 34 to 36, or whatever it takes to keep it out of detonation at about 3600rpm and over. More is not better. On the street you will not notice if you are 2* less than optimum. But your engine will complain about just 1* too much.
Finally bug out the rate of advance.
After that you can attack the Vcan optimization.