Well I can’t argue with that. You have a point.
There still a means for Shuuya that doesn’t show just to everyone including the Priestess his secret. His a Vampire that no holy water or power would kill him so that secret of his is hard to find out.
In fact the Mirrors are some sort a replica in lower tech compared to the scattered ‘Stargate-shaped’ structure all over the world. In the earliest chaps he stated along with Rollo before she became his familiar that the gates indeed operated as gateway/portals to another dimension and worlds.

The mirrors aren’t as big a deal as you think considering that they are only a few (most of them not in MC’s possession or practically usable), there are other skilled practitioners of space magic (even if rare) and magic item makers available, and it can only be used by someone with the space attribute (or was that only the item box bracelet thing?) and even if the space attribute isn’t an issue it is still an issue that only 1 person can hold the key that activates it. Not to mention that the person who made the mirrors must have been immensely skillful in space attribute and magic tools (much of the relevant knowledge has probably been lost), thus making the reverse-engineering prospects dubious at best (this thing is probably older than most, if not all dragons considering that the wasteland dragon “witch” lady has had one for thousands of years with no peep at all). And even that person who made the mirrors, probably couldn’t mass-produce them (or he didn’t want to); even if he made several individual sets (many mirrors that are separate and with their own keys), they are still rare like an extinct species. One more thing, the mirror cannot transport anything wider or taller than the frame, which limits mass trading and transit to small items. All in all, more reliable and easier to use lower-tier tech (like the transport formations made by the spiny-tail demon chick that MC killed) made by others.

Finally, that priestess is so devout, that having her understand (which shouldn’t be hard considering that teleporting greatly shortens the time) what a secret it needs to be wouldn’t be hard in the slightest. Yes, there would probably be people who would want to take it from MC, even with all the requirements and limitations, but that would more likely to be used for personal use or dispatching spies and special agents to collect info as opposed to risk damaging it to futilely reproduce ancient/lost technology. Some of them may already be destroyed as I remember that there was at least one (believe one of the lower-single-digits) that didn’t open no matter what: I think it was destroyed, too far away (like on another planet or dimension far), or in some kind of barrier or seal that prevents it from activating (implying someone knew what it did and decided to keep it closed as it cannot be used without the key, assuming this is the case).

No he can’t show that even to her about his secret mirrors. I mean this mirrors is totally a big deal that would reshaped that world transportation as some sort of a business trademark. Beside the mirrors are his and only chance of getting a shortcut to distant lands that some place are harder for anyone to reached and made it out alive.

Thats a fast friendship..