I have been using the Lioncast LM30 for quite a long time and it might be just the right thing for you to check out. It has 12 thumb buttons, adjustable weight and has proven itself to be very reliant (at least in my experience). Hope it helps.
yeah i got that one and use their keys quite often. especially that button next to the left click is awesome for chatwheel and stuff. Although 6 buttons on the side would be enough. Sadly the steelseries mice only have 2 buttons :/
Maybe the Lioncast LM30 is for you, although it has 12 Buttons on the left.
https://www.amazon.de/Lioncast-Gaming-programmierbare-Tasten-LED-Beleuchtung/dp/B00MQZE1RK?th=1
I have tried a lot of gaming mice for various MMO's (WoW, Aion, Tibia, some korean ones), i kinda dislike logitech because of the quality of the products and im fed up with razer because of the faulty software, so i wanted to try something new, ended up with this:
https://www.amazon.de/gp/product/B00MQZE1RK/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
been using it for a month now, yes i know, not long to make any judgement, but for the price-quality ratio it feels amazing. You can customize everything and the feel is good too (i have large hands and fat-ish fingers, still can use all of the 12 side buttons with no errors. If you are low on cash, give it a try!