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Front shifting works and doesn't happen very often so I see no point in changing it.
Rear shifting happens a fair bit and I'm getting tired of the broken cable stabbing me. Also, Di2 rules when it comes to not fucking up your wrists. I guess it wouldn't be a huge amount more to get both but I was trying to keep it cheap as.
Is the external stuff any cheaper? I know it's going on a shit bike but I prefer the cleaner appearance and protected nature of the internal setup.
10 speed and 11 speed chainrings are the same width - the internal width of the chain is the same, it's just the link plates that are thinner. You could run an 11 speed chain on your 10 speed drive train without issue.
If your front shifting is shit then that's the biggest reason to go Di2 in my view- it's the USP, as come what may, come hell or high water, you hit the button and the chain will move chainrings at the front. Or the whole frame will explode.
I'd go full rather than a cable/electric mash-up, and you can do a very effective install using helicopter tape to secure the cables to the outside of the frame - under the chainstay, the underside of the DT etc. No need to get all driller-killer.