Reading Lure of the Expanse I found the insane statistics for the Zayth Landships and their guns.
Though the guns have the same damage their great secret is that their guns are not man but machine loaded, so quicker which result in a higher Strength.
So what if the Explorers would be able to trade in these guns or use them theirselves.

Zayth Archeotech Macro Batteries :
Location: All ships, all locations
Power: 5
Space: 2
SP : 2
Strength: 6
Damage: 1d10+2
Crit Rating: 5
Range: 5

The power is a bit higher as regular Macro's because of the additional machines, the strenghth is higher because of the increased RoF.

Santiago...

If I read that right, it's basicly uping the strength of a regular macrobattery to that of a Broadside. Seems reasonable to me. Naturaly you'd have to change something if you were upgrading a Battery Broadside instead of a standard Battery, but the principal is sound.

If you think that is crazy, wait till you see the XXXX with 3 dorsal weapon mounts. That's same number of weapon mounts as the Dauntless class Light Cruiser.

Then there are the XXXX with 40+ on every single physical stat. Somebody should tell the Space Marine that we have a new recruiting ground for them. And their "hull scavenged spear" is more lethal than a Latheforged Mono sword.

If you think that is crazy, wait till you see the XXXX with 3 dorsal weapon mounts. That's same number of weapon mounts as the Dauntless class Light Cruiser.

Then there are the XXXX with 40+ on every single physical stat. Somebody should tell the Space Marine that we have a new recruiting ground for them. And their "hull scavenged spear" is more lethal than a Latheforged Mono sword.

Zaythi broadside (taking a month for them to finish a single one after they are supplied with the material needed) should have like 8 strength?

Now it makes sense why one would go to the effort of building a new trade route with zayth instead of just getting guns from already existent routes coming from the lathes.

A full broadside of Zaythi macrocannons being essentially a macrobattery with the rate of fire of a broadside makes sense, and as noted, isn't too overwhelming.

A trade route with Zayth is still worthwhile; you don't need to be shipping entire cruiser broadsides all the time for it to be profitable; a single macrocannon is worth a fair amount of thrones - especially since fighting ships will lose guns (especially in the expanse), and whilst the quantities are lower (you're entirely correct that it would take a month or more to build) you'll still be more easily able to provide (say) a damaged Damaris SDF ship with two or three replacement guns from Zayth faster than you could haul replacements all the way from the Calixis sector.

Plus, some people like having access to naval weaponry that the Mechanicus and the Navy don't have listed in their records. For purely legitimate reasons , of course.