Years ago , a tech member by the name of BenzMac, stated that the boots not only deteriorate from outside but also from within.

Superficial 'cracks' or 'spidering' are just that.
Superficial.
Don't be pressing on them or spraying stuff that has solvent based vehicles.

You can spray 100% silicone spray into a plastic cup and brush it onto the boots.
That feeds the rubber and helps prolong it. That is what I do.

Don't spray directly because you might get it on your brakes and also the propellant or spray vehicle might not be the best for your rubber.

Some scuba shops sell 100% silicone pump bottles for maintaining masks etc..

Brush it on.

You can try GACO 100% silicone paint.
That is roof paint.

Just wash boots lightly with dish soap and let dry.
Then brush on the GACO.

The good stuff might not be available in CA. but the water base stuff yes.
Put it all into quart size cans for storage.
If you leave it in a gallon can,it will probably dry up.

Some members here rebuild them. research archives.

"reinventingthewheel' is operated by a former member.

BTW some axle shafts can be specific to your model.
Some aftermarket have 'proud' hubs that do not allow proper setting. you have to grind them down.

Don't sweat it.

Some oldies last forever. Some don't.
road salt reduces longevity.

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD