Backfill is probably a good idea, keep those WCs though, you can still use them against Boudica, with Catapults, their chances even against Longbows aren't that bad (sounds ridiculous, but 2 against 1 still works, it's only 5 agains 6, and the WC ignores the FS of the Longbow) .
But what are you doing
Revent? Working a Plains-Hill-Mine instead of Wet Corn, just because of Happiness? I hope you're kidding, just let the city grow into

, and then 3-pop whip a Library or build Settlers!
Well done on the rush btw., seems my advice worked. You're basically already in a winning position now, if you don't do things like those with the Plains-Hill-Mine, just settle good cities now, and see, that you get that Great Prophet for Theology, Theology should be enough Tradegood to get you all other techs, so no research needed. If you got too much

(didn't look) so can research, save it and continue with Paper right afterwards, then you can Lib -> Cuirrs at 300 AD. Try to play this step by step, it's not clear, when war against Boudica will be advisable. Could be in a few centuries, when you got your new cities and they're size 4 and have Granaries, could be that you're already at Cuirrassiers then. Just don't settle bad territory, and coordinate your Workers well, and you're good. Also, try to get a GS paralell to the GP, a bulb of Education is helpful always, and an Academy (weaker choice) wouldn't be bad aswell, so think about getting CoL and run Caste, also gives you faster Borderpops via Caste Artists. You can switch to Slavery and whip the Granaries, then switch back to Caste again. Don't build too much Infrastructure, even if you're CRE, and Libraries are half price. You need Granaries, maybe Forges and then you want to continue the war onto the next target, you can still achieve a lot with a mixed Stack of some Elephants, your leftover War Chariots and some Catapults. Try to not waste more time than needed, in war-games it's best to keep conquering continuously, it's just not advisable atm., because Boudica is so far away, and you got so much room left. Once that is filled up though, the next war should follow, not mattering at which unit you are, Elephants and Catapults work 'til Rifles, so no need to hold back, should she have Pikes or whatever at that point, though I hope, she won't, because Pikes is somehow the edge of Elepult, it works, but it's not really efficient anymore.
Good luck.