How can I prevent lead free solder from ruining my soldering iron tip? Can I re-tin my tips?
Your routines are sound, I do the same as you except the #5 - I don't use the copper balls but they shouldn't damage the tip.
Most likely it is oxidation, which will contain flux and various other chemicals that stick to the tip, in which case a sponge might not get rid of it all.
There's something called &quot;solder tip cleaner/activator&quot; that you can try. Basically a little box with some cleaning chemicals, sold by Weller among others - check this video. This isn't something I use every day but mostly when picking up an old tip that hasn't been used in a long time.
Apart from that, it might of course be some issue with temperature or forgetting the station on etc, which also turns the tips gray and useless similar to what you describe.
Also, some 3-4 months of frequent soldering is about what I would expect before changing tip. If you haven't used the iron much then it should naturally last longer.