Nope, you have it. A few more LED's will be all. More current is a 'tweak' level of change. 6 or so LED's and another socket will be all that is required. You'll be pulling a little bit more heater current. Still going to be 12.6, but at 0.6 A instead of 0.5 A. Some tweaking of the R's in filament supply might be needed if you want exactly 12.6. 12-13 ought to be fine IMO...
The 12B4's are mu 6.5 and until you increase heater current or get octal valves, it is the best. Maybe the best without caveats.
regards,
Douglas