[...] you must incentivise people to spend their tokens as much as they can.
one dynamic to consider that helps with "spending" (vs "holding") that goes with a crypto that doesn't reward spending directly is: to the same extent that holding is rewarded, so too is the motivation by merchants to give discounts to those buying with that crypto. So in theory, spending will be encouraged indirectly because merchants will want to hold that crypto compared to other currencies... and so on throughout the digital economy. Merchants will have many other reasons besides "interest payments" to give discounts to buyers, too. So spending Radix probably will happen naturally, IMO.
I think it is impossible to encourage spending just by merchant rewards. First, you need merchants to accept technology (it won't happen soon). Secondly, it is impossible that merchants will hold any kind of crypto just for speculative reasons. If you pay with BTC, actually, they are converting it in fiat instantly! Crypto is not in competition with fiat IMO. For everyday use fiat is the best and it will stay.