I just found out the shower in my 50-year-old house has been leaking
for a while; the shower is directly over an overhang so I found out
about the leak just recently when I saw water dripping from that
overhang outside. So the plywood from the subfloor down to that
overhang is rotted. The beams under the bathroom look ok, a little
black where they meet the plywood. The floor in the bathroom seems
solid structurally, no shifting tiles or cracks.

Option #1 for repair is to rip it all out and replace the bathroom, new
subfloor, probably choose a fiberglass shower instead of tile to avoid
having to caulk the seam of the tile to shower pan.

I was thinking maybe option #2 would be just to leave everything in
place and build an insert that would fit snugly in the existing shower,
caulked to the shower walls. This insert would have a slightly sloped
floor down to a central drain funnel, and the bottom of this funnel
would hang inside the existing drainpipe. This way, water would never
touch the shower pan or the joint between drainpipe and shower pan. (I
suspected the leak was at this joint between drainpipe and shower pan,
and sealed it with putty epoxy from Home Depot. When I watch from
below when the water's running in the shower, the outside of the
drainpipe was dripping before my fix and seems dry now, but there's
still water dripping from somewhere else.)
This insert would just be a wood platform on stilts sitting on the
shower floor, with this drain funnel in the middle, and with vinyl
laminate or something attached to the wood platform to protect this
wood from the water.

Any comments on option #2, or suggestions on other options? I'd really
rather not rip out
the bathroom. Thanks.