First,I have zero experience with an LC 9.Its possible I'll make some incorrect assumptions.
I assume the barrel has a blue or oxide finish.The whole barrel goes in the tank,they don't "mask" the bore.
Lead bullets aren't going to wear that off very quick.But I would not ,myself,work to accelerate the "wearing off"
To a point,it matters how soft the lead is.And,if you have found some kind of lead factory loads,or lead bullets from major bullet manufacturers,they are typically smeary dead soft lead.And they do lead.
I have fired a lot of wheelweight full power loads through a Ruger Super Blackhawk.They were higher velocity and pressure than you were likely using,with no leading problems.And it was never a a lapped/polished bore.
I'll tell you what will affect both accuracy and leading with cast bullets.Bullet fit.Particularly,undersize bullets.
Unless you slugged your barrel,found out your groove dia,and are shooting bullets sized about .001 oversize,you may have a problem.Undersize bullets are a problem.
To start with,the 9 mm bullets have a short cylindrical bearing diameter.Undersize bullets tip,and never get pointed straight.
Another problem,undersize bullets do not seal the bore.This lets hot,high pressure gas jet past them.It melts the lead,and this is commonly the root cause of leading.
I won't say fire lapping never helps a barrel.I have done it to advantage.
But I do it when I figure I have an otherwise hopeless case ,with nothing to lose.
Myself,I would not use the steel brushes.To each his own.
There are some pretty good solvents.Look for one good for lead.Strong copper solvents eat bronze brushes.Maybe lead,not so much.
Once you get it clean,try some jacketed bullets for accuracy.
Lead bullets can be wonderfully accurate,but they have to fit.