Well, personally, I would say there is no need to buy missiles that often - in fact it's very wasteful if you're free to play and need to spend your donuts efficiently. Those 2 donuts a day would be better spent on the final energy refill as that will save you a lot of coin. If you don't have time to spend that much energy... well, the 1st refill is not worth buying with donuts in my opinion, but if you have time to get to and use the 2nd or 3rd refills, you could spend the 2 donuts on whichever one of those you have time to use instead.
Though there is nothing wrong with enjoying PVP in itself, the coin from it is pretty negligible especially later on (and you can still get good coin from duelling without using missiles) so the main reason to duel with missiles is honour. Honour is very easy to obtain when you are strong and high level, even if you don't ever use missiles when attacking people, and therefore I think it would pay off more in the long run to spend those donuts on energy refills to improve your character instead of on missiles to help you accrue honour in the short term. You will soon enough hit a ceiling where you can barely gain any more honour even when using missiles constantly, because you have overreached your strength and have now become an attractive target to far stronger players higher up in the rankings. You can still use missiles for your defence, for PVE and for team fights, but if you unequip them before attacking other players you'll find you don't need more than 1 or 2 sets of missiles a week. Even if you don't have the time to get to the other energy refills, that would save you a lot of donuts which you may need later when donuts from deeds and such start drying up. The only good reason I can see to duel with missiles constantly is if you are trying to do well in the honour tournament and win more than enough donuts to cover missile costs. However, that tournament is mostly dominated by high level and high stat players so I don't think you'd do very well currently.
Also I would caution you against investing donuts in a sidekick at your level. Since the sidekick's stats are based on your character's level (I believe the formula is something like (your hero level)*3 +/- 25), for the best stat boost you will need to replace your sidekick occasionally. And being as low level as you are, you are levelling up quite often and therefore will soon outgrow your sidekick. If possible, try to get your sidekicks from villains instead - it will cost you time attacking it and a sidekick is not guaranteed but if you do get one, it's totally free. A sidekick from a mission or possibly bought with coins from the shop would be a more likely alternative, though if you could put up with having an outdated sidekick for a while, that would be most efficient of all. Once you near the soft cap at level 249 you will rarely need to replace your sidekick anymore and at that point a sidekick bought with donuts would be a better investment than before. Near soft cap is also a good time to start focusing mostly on missions that give a lot of coin.
Of course my advice is all moot if you are happy buying enough donuts to cover the costs of missiles and sidekicks, but since you mentioned trying to spend your donuts efficiently, I figure this is something that concerns you. Still, you've obviously put a lot of thought into your play style so perhaps you have already considered the points I've raised. Otherwise I think your strategy is pretty good.
Apologies for the long reply 
