I think there is some kind of misconception.
Tags are not selected by Itch. They are selected (and created) by the developers. There are even typo tags. Like Horor.
So Itch cannot "merge" tags. Or rather, I doubt they would want to. There is no fixed tag list, like on Steam, where those limited tags are also chosen by players or maybe curators.
What Itch does, is to manage a "suggested" tag list. It is the content of the drop down list in that tag box. But that list is by no means a collection of the most used tags. And not even all those suggested tags have a description. And those descriptions that do exist are neither set in stone nor binding. Basically it is like hashtags. A developer writes the tags that come to mind - like https://itch.io/games/tag-banana . Or uses whatever seems a good idea to describe the game or to help people find it. Mileage varies a lot here, hence your post.
But what they can do, is make aliases. Like, "commodore 64" is aliased with c64 and commodore64.
So if you see tags that really are the same, you can suggest those aliases to Itch. The different tags will still exist, but searching for one will give results for all of them.
You could also write a guide posting in https://itch.io/board/33833/itchio-tips how to select relevant tags. I mean, some 40k games even have this tag: https://itch.io/games/tag-indie
But some things what you suggested are not quite the same. Enthusiasts of those nieches would probably like to have a word with you, for mixing their thing with those other things. To give a blatant example, people looking for gay content would not be happy with finding lesbian content. And just because you look for norse mythology does not mean you want to find greek mythology.
Others might be the same and are synonyms, so an alias would make seldom used tags more findable.
To pick up your last thought about the tag "suspense". What does it mean? It means the same as every other tag. It means whatever the publisher that selected that tag wants it to mean - within limits of the language. If suspense is something that can be associated with the game, it fits. If it is in the top 10 of tags that would classify the game, is another question.
And different tags for the same might also be a flavor thing that could have impact on recommendations. Like chosing flight over flying and vice versa might indicate something. Whatever it is.
What I would like to see is aliases for the engine tags. There is an Godot tag in usage. But also a made with Godot meta-tag. So if you search for godot you miss out on the games that did not use the tag, but did specify it in the meta info. And if you search the meta-tag (if you stumble upon it in the Diretory in the footer) you miss out on the games that did not fill out the meta info, but did write it as a tag. About 10k games that have the godot tag are not classified with made with godot. There are 53k madewith, 23k with regular tag and 13k overlap. So making the meta tag games appear via the regular tag would be nice for browsing. One would find about 63k games instead of only 23k.