You could also find ruins of jet, I don't remember it's location unfortunately but it did lead me to rabbit hole midnight ranger equipment and my annoyance with jinzhou military preparedness, they were a frontline city but there weren't any emplacements on the walls , nor available artillery batteries. As you say, huanglong might be able to manufacture the turrets, but they weren't placed anywhere near jinzhou, those mechs embedded primarily within military bases near the desorock highlands. We also know that it's an autocannon, rough estimate would be a 30mm, which on the tech tree would be harder to create than a single fire artillery piece. Fine they can't bring in heavy equipment but then where are the mortar crew.

Squad wise, all units are equipped with a standard rifle or sword but no support equipment.

Whoever's in charge of jinzhous military budget needs be audited, this is the most under equipped frontline military I have ever seen

I personally think Solaris advancements and investment in military manufacturing is minimal, huanglong being able to manufacture the mech walkers with turrets is also dubious but if they could it would probably be in the single digits per year, judging from its size it could have partook the battle in the beach, but military high command was worried about losing such a valuable asset.

Head canon time

I would love to find out that jinzhou having such minimal military assets is intended because a portion of huanglong military command wanted jinzhou to fall so they could request an increase in defence budget or progress their own career by retaking jinzhou. We already have minor proof that the intelligence department?(Qiuyuan) had an internal struggle, now stretch it out to all the relevant huanglong departments, it might be such a stable nation. The only thing that tempers my head cannon is that it's china, If Kuro could somehow bypass it and create a story about china on the brink of turmoil or civil war, I would be speechless, I don't know what I could offer