EDIT to answer replies:
@3phaseeel 30-amp wires are 6 AWG aluminum, 50-amp wires are 4 AWG aluminum. The sheathed wires run along the garage roof truss overhead (see picture), exposed for about 28 feet in length, then in-between insulated joists and down a wall, then exposed again in the enclosed crawlspace. I added a picture of the panel door, not sure if the sticker from 1982 adds valuable details.
@manassehkatz Subpanel will be used for dedicated circuits to bathroom heaters, not for heavy-duty usage like welding.
I have an unused electric furnace that is serviced by one 30-amp double pole breaker and two 50-amp single pole breakers. I'm thinking of:
(1) reusing the existing wires from the 30-amp circuit to service a new heat pump with a dedicated 25-amp replacement double-pole breaker
(2) reusing the wires from the two 50-amp circuit to service a 100-amp subpanel to serve future wiring projects
Does that sound like a reasonable plan? Or is this plan not as simple as it seems?
We switched to using a mini-split heat pump two years ago, which cut our electric bill in half, even when used in combination with a few space heaters. Having had good experience with the mini-split, I now want to install another unit to completely replace the space heaters and retire the unused, noisy, leaky, 38-year old furnace. I am a DIY homeowner with advanced beginner to intermediate level of know-how and tools.