using 265/70/R18 as a spare for 285/70/R17 tires

Have a 2025 Tacoma on the way, and before I pick it up I want to ensure I determine whether or not I'm going to keep the OEM fitted trail terrains. If I'm not keeping them, I'd like to take them off right away to re-sell them at a higher price. Side note, I have winter specific tires ordered for the truck already.

I am looking at using Toyo AT3 285/70/R17 117 (non LT) tires on the OEM rims because they are lighter than the other options which should give me more fuel mileage but more of a trustworthy off road tire for when I am way back in the sticks without reception.

I will be keeping the OEM spare for winter use to go with my winter tires as they are the same diameter.
However, in the other three seasons, I am considering buying an 18" rim and installing a 265/70/R18 AT3 (non LT) on it so that it doesn't hang so low in the spare tire location... I'm assuming this shouldn't do much arm to the vehicle if I had to use it? When the tires are new and unused they are 0.2" in overall diameter smaller which honestly could end up being the same diameter if I have to use it given the main tires are going to wear. Is my theory correct?

Two additional notes:
- I will not be doing a 5 tire rotation
- use case is 90% commuter, 10% heavily eroded old forestry roads with sharp rocks all over (this truck is replacing a car and a sxs)