WW231 and 125 grain cast bullets... Great combo in my experience run in my Beretta M9. Are you actually weighing the thrown charges? If not you really need a scale to check your charges, as this will allow you to really dial in safely the proper charge you want to use. Also as you wisely mention, obtaining a few reloading manuals would serve you well.
That said, I usually go with 4.2 grains of WW231/125 LRN, and my chrono says right around 1060 fps for my lot. Accuracy is good, but truth be told my best groups with 125 grain LRN bullets comes from Unique. As with Kirby, my barrel slugs .3572", and I usually size my self cast bullets to .358". May not be a bad idea to ask the person you buy bullets from (maybe you cast your own?) to try a sample pack of your bullet of choice sized .357" and .358" to see what it likes best. If the round will fully seat easily with the proper OAL, then the diameter is OK.
I think one of the biggest problems for folks who load lead for European branded 9mm pistols is going with the defacto .356" diameter... Hell, even the jacketed .355" is not the right fit for a lot of pistols, and I load .357" jacked with very good results. As for 124 grain Speer JHP, you will see data all over the map as to what is a max charge. Consider a worked up, weighed charge of 4.8-5 grains as a max std velocity round, knowing some data will go a bit higher. Personally when I want to launch 124 grain JHPs/FMJ's to 1200 fps+, I reach for Unique or Power Pistol.
Good luck!