I cant tell if the joists are eaten, but, that you don't fall through leads to the supposition that termites are not involved
I use a laser to mark from the highest point, a level mark on all the other joists bearers and stumps
its cool how far, in the half dark under there, those level lasers project,
eg: the light is installed 20.0 below the highest joist,
is 12.2 below another joist,
jack each point up till the light is 20.0 below the joist, block it, move to the next one
I have a piece of plastic bar, with green ink on it 20.0cm from the end, red laser light makes the ink fluoresce like a 100 watt light, easy to see when the height is right
get done, perfect, floor no longer bouncy,
the most difficult part is access to the underfloor area, as posted before, it is sometimes a circular saw through floor boards
I find it easy,
but: (famous last words but:
)
I have 12 tonne jacks,
circular saws,
don't mind cutting foundation walls and installing a lintel and access door,
know how to determine which points have, and how much, load on them,
know how to repoint masonry.
can remove and replace and refinish the floor boards and hide the saw marks
probably a carpenter. If you want to TA for the chippie, its a learning experience, and there is always a 'next time'
If you are opening up the underfloor,
its a bloody good time to get a plumber in to replace any gal pipe with pex before it rusts up and stops running or bursts, the age of the property suggests gal,
gal is perfectly rust resistant, until the last of the sacrificial zinc is absorbed into the water, then it rusts out fast
pex over copper,
flexible and you can pull it through walls without issue,
needs no holes except the ones the fixtures are in,
has equal pressure tolerance,
does not go green and dissolve in contact with mortar,
needs no flame to join (dead dry old seasoned timber),
is half the price,
takes less time to install