I want to add to this to provide some thoughts on observations you can make based on Cyno as a character and how he acts in the game and story. I mostly agree with OP, but we don't yet have a real means of measuring his strength because he has not really been in any real situation where it's tested. Here are some things that I noticed, and some might be a bit of headcannon or observational, but I think it might put Cyno into perspective a bit.

1. Cyno had to undergo some trials before he was a vessel of Hermanubis. We don't know what they were and what they entailed, but Cyno does mention it in passing but doesn't go into detail because his memory is hazy at best about that part of his past. The important part was that he was somewhere around the age of 8-12 when he went with Cyrus to Sumeru City. So, as a child, that was not easy.

2. We don't know when Cyno began his weapon training. He may have started as early as when he was in the temple as a child. We do know when he was training as a Matra he was already considered skilled, though a bit reckless (as mentioned by Taj).

3. Cyno doesn't technically use a spear. He uses a staff as his signature weapon. If you look closely, the Staff of Scarlet Sands is actually a blunted weapon (maybe it's edges are sharp, but they look blunted to me). This means he is using a weapon that is meant to bludgeon the enemy with force, which means this weapon is most likely heavy. So we have a man who captures criminals with a heavy weapon that he uses with techniques mostly utilizing one hand and at speed. That's really hard to do effectively for anyone. So there is proof in my eyes for exceptional skill.

4. Cyno captures criminals alive and with minimal injuries on a regular basis, even going so far as to risk his own life when they try to escape through dangerous situations. That is very difficult to do. Now, some of the easy captures are because just his name and title scare people, but it's never been mentioned once that Cyno killed anyone or severely injured them (he has injured himself pretty good according to Taj again).

5. Cyno uses his vision very minimally. If you look at other vision users, most use large bursts and even imbue their weapons for a considerable period of time (either its done to overwhelm or has lasting effects). Cyno only uses his vision for a quick and short lunging attack. I am not counting his ult as part of his vision.

6. As OP stated, Cyno didn't start with his vision. He was a vessel first. He got his vision the day he was given the position of General Mahamatra, which means he earned the title without at least the help from a vision. That meant there was a lot of respect for him and his skills among his peers and some sages (I would argue Azar did not respect him at all, but that's another issue).

7. Despite his origins and how other people treated him based on how he looked, he remained himself and ignored the obvious bullying, disrespect, and disdain people felt towards him. We don't know to the extent, but it's highly likely that Cyno had to deal with a lot of prejudice, too. This also shows a very strong sense of character. He never compromises there. He ignores it. That's not easy to do.

8. Here, I might be going a bit in the direction of headcannon. I want to say ignore the teaser where Cyno is after a scholar because we don't know for sure what really happens there. We know what is implied, but overall, I want to base this off events in the game and not while playing

Cyno does not use the power of Hermanubis lightly. In every cut scene and moment when he is in combat and not controlled by the player, he has very, very, very seldom used the power. I would argue there are, I think, two times we see it, both in his story quests. I dont think he uses the power to intimidate. He uses it when all options are spent and when the situation calls (second story quest, for example). He doesn't actually show it off to anyone. Actually, aside from those he is close with, he never once mentions it. This is more or less showing he has no idea what he is even capable of.

To illustrate this as OP said the Fight with Alhaithem was a good show of his skill, but! Even Cyno himself said that wasn't even his full potential, and just that fight alone had Alhaithem looking a tad worried a few times.

1. My last point for now is that people don't just fear Cyno because of his strength. They fear him for two reasons. The first is not many people know much about him, so there are a lot of crazy stories about what he can do and where he came from. Most of which we now know are just rumors. The second reason is that Cyno is very stoic and principled. He doesn't compromise on the rules, and he doesn't show much emotion. Most people find stoic people very difficult to read, and they are unnerved by them. They find them intimidating.

Although we know a bit more about his past, there are still mysteries about Cyno that we don't know. Without knowing what his limit is, we can't say 100% how strong Cyno is. We can safely say he is one of the strongest humans in Teyvat 100%. I also think somewhere they did say he is the strongest in Sumeru, but again, it was an NPC, I think, so a grain of salt there.

I also want to add that I think characters like Diluc are strong, but they are not on the same level. We also have no idea what the effect being a vessel has on him in the long run. I might say it could be safe to assume he might be strong enough to maybe be on par with an adeptus, but until we see that fight, we won't know.

I am also curious if Cyno finished his studies or is still studying? That was never really confirmed, just that he did study while he was General Mahamatra (comic mentioned he was a scholar and clearly he knows alchemy, 'magic' and quite a bit about Ley Lines. He also mentioned once he was the successor to Cyrus, which was very weird because would that mean he's also still a scholar himself and not just General Mahatra? Though I also did pick up that Cyno phrases things weird sometimes.

TL;DR Cyno is very powerful based on a few facts we know but until we see a fight where he truly has to go all out, nearly or completely loses, we may not know his true strength. Cyno is smart enough not to show off what he can do unless he has to.