Whenever implementing features, the only question a designer must
always ask is: "Is it fun?"
If a feature is not fun, then what is the point in implementing it in
a game?
What you are suggesting is called Balance Compensation and all successful
studios stay away from it as if it was nuclear waste.
While it is true that mining must not make a game too unbalanced,
the features that define mining (or any other aspect for that matter)
must not be set in place to limit a player, but to create alternative routes
from which the player can then choose from.
Instead of tweaking mining, the whole underground implementation
should be re-designed.
The UG should be in the same "mapspace" than the surface world.
Meaning, you don't need cave entrances or exits - you just walk into
a cave, if there is one.
This also means, that when you are mining underground and the ground
on top is in declining slope, you will reach the surface level when digging
out the cave wall that is one height level lower than the surface level.
This causes seamless transitions and enables monsters from caves enter
the surface world. This in turn enables night-prowling monsters to exit
caves at night and is a good feature for a horror survival game.
Mine veins should be generated once when the server generates the world.
After that, when an ore is mined and cave-in caused, the tunnel is filled
with the surface material the ground above has, meaning that the topmost level
cave-ins will be dirt, which can be easily removed with a shovel.
Mined ore is not regenerated with a cave-in.
Now, there should be multiple UG levels. Maybe 10, but should be easily configurable
in the server cfg. Having multiple UG levels increases the
ore available to the player, but, as the depth increases, the rock density increases,
and the player has to use better tools (and maybe more time)
to excavate deeper levels, making the player carefully consider if he should
mine horizontally in the easy upper level or mine down vertically into more
tough rock for, say, diamonds.
This system also means, that it is possible to create open-pit mines down
to the most bottom level.
Also, when there is a cave-in at lower levels, all of the unfortified stone above
will cave-in and cause the ore in, say, level 5, to fall down to level 7, if
that is where the cave-in originated.
This also allows to cause damage with earthquakes and other natural disasters
like volcanoes.
In conclusion, the whole underground levels implementation should be redone,
instead of just tweaking the classical system hoping for a better result.
The current classical implementation is sub-par for RPGWO reincarnate.
Also, this system provides enough ore, if the player is willing to put effort in it
and makes it tougher to mine new ore.
Also, to prevent server resets, ore veins should regenerate in land plots which
have not been mined for a long time, say 1 real time month.
This allows new players to find ore in the easy upper levels of UG, if they
wander around enough.
If there is going to be a newb town or island, the ore should always regenerate
for training purposes.