I'd go with the timing. If you raise the compression, the burn will be quicker/more complete - which means you need less advance (a little less initial and total, so you can get away with just dialling in a couple of degrees less initial) - if you kept the timing where it was, it'll be doing a little bit more 'negative work' as it burns and expands before the piston has passed over TDC, so a little less advance gets this 'push' happening when it should. 2-3 degrees should do it (if it took more than 4 degrees I'd be surprised). It's also worth noting that it can have too much advance but it still won't be audibly pinging at lower rpm.
I'd hold off altering the mixtures for the time being. Going richer only lowers the temps if you go richer than stoich. This would only be called for if it's really bad at full throttle. When you are running part throttle, you aren't getting anywhere near the same effective compression/volumetric efficiency, so going rich would be costing fuel, but not actually a major benefit, as you (shouldn't) be near detonation from mixture alone. It gets even more interesting, because as you go leaner, it does run hotter, up to around stoich a/f ratios, then when you lean even further from there, believe it or not, chamber temps actually drop off again. Believe it or not you can actually run cooler at part throttle by going leaner than stoich.
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