Don't forget Kingpin's idea of switching to a petrol motor to drive your mixer at variable speeds. If you can tolerate the noise, fumes and increased running costs, it's a proven, off-the-shelf solution. A petrol motor could achieve the 67% speed reduction capability you seek, if it could be throttled back from say 3300 r.p.m to 1100 r.p.m.
I mention this because if variable-speed electric motor drives for domestic concrete mixers were problem-free and cost-effective, why aren't there more of them out there? Getting one to work reliably might perhaps require time, money, and experimentation.
