Moto G5 (not sold here, but available on Amazon). Dualsim, band 2,4,12 but can't do VoLTE. Bootloader can be unlocked. Perfect for your project. Don't know second hand prices though. https://www.amazon.com/gp/product/B06XH4396B/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
The G5 is $225 on Amazon while the G6 is over $300. G6 has similar specs to the 7X while the G5 is underspecced at 2 Gb of RAM. I'm inclined to go with the 7X or Redmi 5, after I figure out if the global version they're selling work in Ontario.
The Mexican/Latin American version (xt1671) has NFC and dual sim and works on T-Mo and ATT in the US.
The con is 2/32 rather than 4/64.
Yeah, you're not going to get a better plan without dropping to something like Wind/ Freedom. I'd stick with your plan if you're happy and upgrade your handset.
So your list will vary depending on who you ask, but you should be able to get a nice handset for $400. I would suggest the Motorola/ Moto brands for a near stock experience, but be aware that most lower end models come with 16Gb stock (you can add an SD). Sony handsets are also pretty close to stock, but YMMV with them.
Edit: So I just did a quick scan through amazon. You can get a new model moto g5 for ~$350 or a moto x play for ~$450.
I was referring to the G5 international version on amazon. https://www.amazon.com/gp/product/B06XH4396B
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ $167
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ $167
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ $169
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ $169
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ $163
Nokia 6 - Prime Exclusive. 5.5" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 16 MP, 3000 mAh. https://www.amazon.com/dp/B079HBYFH5/ $199
Moto G5 Plus. 5.2" 1080, Snapdragon 625 2.0 GHz Octa, 32 GB/ 2 GB, 12 MP, 3000 mAh. https://www.bestbuy.com/site/motorola-moto-g-plus-5th-gen-4g-lte-with-32gb-memory-cell-phone-unlocked-lunar-gray/5762700.p $209
There are a couple new options that are near that price range and are somewhat small.
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ref=cm_sw_r_cp_apa_0ibCAb2EYKKNW $170
Moto G5 Plus. 5.2" 1080, Snapdragon 625 2.0 GHz Octa, 32 GB/ 2 GB, 12 MP, 3000 mAh. https://www.bestbuy.com/site/motorola-moto-g-plus-5th-gen-4g-lte-with-32gb-memory-cell-phone-unlocked-lunar-gray/5762700.p $229
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ref=cm_sw_r_cp_apa_0ibCAb2EYKKNW $169.99
Ulefone Armor 2 5.0 Inch IP68 Waterproof 4G LTE Smartphone FHD 6GB 64GB Helio P25 Octa-core 16.0MP Android 7.0 FDD-LTE B1/B2/ B3/B4/B5/B7/B8/B12/B17/B19/B20 $244.99 https://m.banggood.com/Ulefone-Armor-2-5_0-Inch-6GB-RAM-64GB-ROM-Helio-P25-Octa-Core-2_6GHz-4G-Smartphone-p-1186091.html
Moto E (4th Generation) Unlocked (AT&T/Sprint/T-Mobile/Verizon) - Black - Prime Exclusive. 5.0" 720, MediaTek 6737 1.3 GHz quad, 16 GB/2GB,8MP, 2800 mAh. https://www.amazon.com/gp/aw/d/B077Y79RBY/ $120
Moto G5 4G LTE International Version (Lunar Grey) 5.0" 1080, Snapdragon 430 1.4 GHz Octa, 32 GB/3 GB, 13 MP, 2800 mAh. https://www.amazon.com/dp/B06XH4396B/ $163
Yep
Moto G (5th Gen) G5 4G LTE Dual Sim XT1671 32GB Finger Print Reader Octa Core 2Gb Ram International Version Desbloqueado (Lunar Grey) https://www.amazon.com/dp/B06XH4396B/ref=cm_sw_r_cp_apa_UoQAAbT5XBBJA
In that case, would this one work? https://www.amazon.ca/Motorola-XT1671-Factory-Unlocked-Lunar/dp/B06XH4396B/ref=sr_1_2?ie=UTF8&qid=1511501121&sr=8-2&keywords=moto+g5 , it seems to have band 4