Xbox Game Pass Ultimate

I've had a hell of a time figuring out how this subscription actually works if you are already halfway through a standard 12-month Xbox Live Gold subscription. Currently, I'm seeing the deal for this Ultimate Pass as being "3 months for $1" which "renews
at $14.99/month". I keep seeing forums/articles describing how you should prepay for up to 36 months of Gold, then convert to Ultimate for "just $1", without much elaboration on that last bit.

More specifically, I just want to try this Ultimate Pass thing without needing to end up paying $15/month. That is simply not worth it for me. But if it's as I'm currently understanding it, with this current deal going on, I'm wondering this: If I'm almost
halfway through a prepaid 12-month Gold subscription at $60 (started in August 2020), and convert to Ultimate now, what am I actually going to end up paying here? Do they just tack on a $1 for each remaining month of my current subscription once it's converted
to Ultimate? Very confused about this.

Right now, I'm basically paying $5 per month for Gold. Continuing with my regular subscription, I'll basically be paying $5/month for the remaining 8 months of my subscription ($40). If I convert to Ultimate now, how much more will I essentially be paying
by the end the remaining 8 months?

$41? (basically just $1 added onto current Gold subscription price)

$46? ($1 for 3 months, then $1/month for remaining 5 months of current subscription)

$76 ($1 for 3 months + 5 months at $15)?

What am I missing here? When does that "$15" renewal actually start? At the end of whatever current Gold subscription I've got going already after conversion, or right after the $1 for 3 months "trial" period?

Or as people have suggested, if I go ahead and prepay for 36 months of Gold for roughly $180, and then convert with the current deal, how much more over that 36 months would I end up paying beyond $180?

:)