You were doing ok, I've tried starting that reaction just dripping in the BzCl/ether at the beginning and usually it would take a long time to start if at all, and only after about 5 to 10% had been added anyway.
Adding 5-10% of the Benzyl chloride/ether sol to start is the easier way especially when your going to start scaling up. I would suggest better cooling through your condenser 0 to -10 deg C Also don't fuck around pulling the flask in and out of the ice bath. Once in leave it. Usually stirring would not have started yet to let the Grignard begin. Don't start stirring until the reaction subsides. Then only slowly still in the ice bath if it gets wild stop stirring and wait. This reaction takes a bit of time to get used to. After a few you will get the hang of it.
The biggest SWIM could get it was around 4lt before the sol would become to thick to stir properly THF would probably fix that. Look at the procedure in Vogel's 3rd on this reaction and benzoic acid prac you follow this and you won't go wrong.
You will get better yields making PAA using Vogel's ratio of dry ice to Mg complex. Use recrystallization with water to seperate the oil form PAA crystals (they look pretty cool). On a larger scale make the carboxylate salt to sep out the shit by extraction.
If your going down the one pot shot by adding methylimine/acetalaldehyde to the BzMgCl don't use ether as the grignard solvent, during the reflux it becomes burnt crusty brown poo. (Again it solidifies and can't be stirred) DEFINITELY use THF it solvates better and the higher temp speeds it up. Goodluck