You have some interesting ides most should be good.
Start with your buck/boost converter (SEPIC). I use them a lot so lets determine what it is going to need to power your motor at full load without any extra for starting etc. The data sheet states “The module also offers a very high conversion efficiency of less than 94% with a 0.5% load regulation. So lets do some simple calculations. I will round the 94% to 85% efficiency, just to be safe.
Start by multiplying the 5V times the efficiency and you get 4.25 (use as a pseudo voltage) We take our target voltage 12V and divide it by your pseudo voltage of 4.25V and get a ratio of about: 2.83. Now multiply your motor current 0.7A times 2.83 and you get about 2 amps. This is what the batteries will have to supply without any additional for stall, overload, etc. Your converter should be adequate.
First many people do things with Arduinos they should not, That does not make it OK. Many times we hear it worked just fine then quit days, weeks etc later. Many times it is they did not follow the rule “A power supply the Arduino is NOT!”.
Lets use 100mA for the Arduino current requirement, there are many varieties and this should be safe. At this point you need 2.1 amps for the Arduino and motor or 2.1 amp hours of power per hour or 6.3 for 3 hours.
The next thing I suggest is putting the motor in a freezer and see what power the requirements are at the colder temperatures. At this point you should be able to determine what you need and please let us know what you decided to do.