I got some very helpful responses on the GURPS Discord channels. One major tweak I'm going to make (mostly as a handwave) to help my gaming group is: The rock-based enemies can be weakened by fire spells. (The rationale is that the dwarves used fire to forge these construct defenders, so fire likewise can unmake them.)
After an enemy has been hit by a fire spell, it loses DR equal to the hp damage that the fire spell would normally have done. Any excess damage from the fire is then converted into duration (1 second per point) before the rock creature starts to regain its DR.
While the rock creature is at reduced stone DR, it is magma-like on its surface, meaning that bladed and pointed weapons can do more normal damage. However, the rock creature will spew forth sparks and a rain of magma droplets if struck, potentially threatening nearby melee assailants with some damage. This means that the swashbuckler PC will get to show off some deft footwork, and the ranged scout/assassin PC can engage in distance sniping.
And the fire mage, while not doing any direct damage herself, will nonetheless be quite important to debuff the enemies so her buddies can get in and start dealing damage to the enemies.
Meanwhile the ST 18 Barbarian is wading into the rock creatures with her Club of Obstreperous Problem-Solving while shouting "Problem???" as she is wont to do.
Edited: session 1 report with map screenshots.
Please note the underground cavern departs significantly from the published adventure to better accommodate the traits of my players and my limitations as a GM.
Here's a
Tabletop Simulator 3D build of the tavern map, along with an attached stables. The Rock Mites milled around the tavern harmlessly and also entered the stables. At one point, a character attempting to escape through the stables tripped over a chest of coins, which the Rock Mites distractedly mobbed, trying to eat the yummy metals.
I also have a
Tabletop Simulator 3D build of the caverns but these depart quite drastically from the published Pyramid adventure. If these screenshots are too unrelated to post here, and thus deserve to go elsewhere, I can remove them as needed.
Here's a top down view of the entire map as overview. Similar to the published adventure, the PCs enter through a well and get to an underground river, which they must cross. This leads to a place of dark power, and defeating the dark power will liberate the overworld from the localized horde.
The well entrance has a bit of setting-specific backstory: the new government has been sending out water purification tablets to most settlements. The innkeeper forgot to dose the water correctly, and then threw all the remaining chlorine tablets in at once before the new caravan arrived. The concentrated chlorine gas caused a weakened tomb wall to finally break open, unleashing the angry dwarf draug's curse ("Leave my land!" which causes a load of Rock Mites and other rock beings to rise and eject the interlopers). Above the entrance way, you can see a red plate with runic writing in it - I secreted 10 of these around the underground system, and if the PCs notice them, they can also stop the Rock Mite horde by defacing all 10 of them. [It's not looking good - the PCs have largely ignored them and have not made inquiries.]
Beyond the well entrance, the angry dwarf draug kept two Granite Wolves (a downpowered version of Obsidian Jaguars) near the entrance, as well as two Rock Mites near the back of the chamber. The PCs took out the two Wolves without much trouble, and then engaged the Rock Mites. At this point it became obvious that the barbarian (shield and club) was having a breeze, but the fire mages, archer, and swashbuckler were starting struggle with the high DR, so I enacted the workaround mechanic described above (fire removes the DR, allowing normal combat effectiveness from "finesse style" enemies). Here's an
overhead view of this room - note that two Stone Golems were visible beyond the rock pillars at the far end but cannot be engaged by the PCs. This was more for a foreshadowing than anything else.
After dusting off the Rock Mites, two more Rock Mites and a Rock Troll came from a side passageway. The PCs defeated them and were ready for a bit of rest. They examined a mural on the wall, which recounted a battle of Groshpalk, a dwarven thane, who held off a human army in a mountain pass. His army eventually prevailed by setting a trap, luring the human army against Groshpalk's elite personal guard, and then triggering an avalanche that killed the humans' spearhead. (Unfortunately it buried Groshpalk and his handful of loyal retainers too, but Groshpalk accepted that as a sacrifice.) The mural ends with the usual admonition "Leave my land".