I don't know who came up with the copper foil charge ring idea but it's a good one, and it's been used by DIY builders for many years, including all of my builds since 2008.
Vapor-deposited indium-tin oxide may be the current state of the art in diaphragm coatings, but that process isn't feasible for us DIY builders, so we have to make do with whatever we can find.
Historically, DIY coatings have included rubbed-in powdered graphite, or graphite slurries (messy and difficult to get uniform resistance), carbon-black mixed with a paint binder (very reliable but opaque), wiped-on dish soap (works well but attracts dust), metalized Mylar (way too conductive, prone to arcing), and mixtures containing anionic salts. Of these coatings, only carbon-black paint has proven longevity.
The DIY community now uses anti-static ESD products almost exclusively (Licron Crystal, Staticide, Stat-Clear). These coatings have close to ideal resistance (E7-E9 Ohms) but no one really knows how long they will last.
I've used LIcron Crystal for about 12 years, but I've given away the older panels and I don't know how many are still playing. The oldest wire panels I have kept up with were built in 2015 and the owner says they still play fine. My personal speakers with the Licron Crystal coating are about 5 years old now and I haven't noticed any loss in output. Time will tell....
The Licron is working well so far, but it's not cheap, and I would love to develop a cheap, easy, clear coating that lasts forever. This would be the holy grail.
To the couple of DIY builders out there who claim to have such a coating, and are selling their closely guarded secret sauce, I say this: Yes of course, you have every right to guard your secret for sale and profit, but you probably aren't making a lot of money on it since the ESD products showed up (there aren't that many of us to buy it from you), so why not share your formula with those who would appreciate your benevolence?
The speakers I build were largely enabled by other builders sharing their knowledge with me over many years. So if I had a holy grail coating, I would pay it forward by freely sharing it with the DIY community.