This is the first time I have eaten sushi and I liked that
that should be possible because have eaten means that my meal is finished but I'm not sure or should use like
This is the first time I have eaten sushi and I liked that
that should be possible because have eaten means that my meal is finished but I'm not sure or should use like
You can say "This is the first time [that] I have eaten sushi " at any time after you have put a piece of sushi in you mouth and before you leave the table/restaurant.
because have eaten means that my meal is finished
This is not true. Even if there is the rest of the meal to be eaten, you have eaten as soon as one piece of food enters your mouth and you have bitten it.
Does "this is the first time I have eaten sushi "mean I have finished eating or I'm still eating sushis?
Neither. As you can also say this after the first bite and immediately after having eaten all your meal, the present perfect does not tell you the stage of the meal you have reached. All we know is that you have started to eat.
The pronoun “This” has a great effect on the sentence. “This” implies the current experience, and that can be understood as being at the table/restaurant or the action of eating a piece of sushi.
1: This is the first time I have ever done something that I am still doing1
2: That was the first time I had ever done something (no 'still doing' implications)
You can't really start #1 with That is..., but you could start #2 with This was... if preceding text has established the sushi-eating occasion as a "current narrative topic / focus".
Equally, you can't use the combination of verb tenses This is ... I had in #1, but you could use That was ... I have in #2. The second verb form is really just a stylistic choice reflecting whether the speaker is more interested in talking about his past or present circumstances.
1 Sometimes you might use Present Perfect like this if you've very recently finished doing something. For example, you might say #1 to the maitre d' as you leave a restaurant, after having eaten sushi there. As ever, Present Perfect simply implies a very strong connection to "time of utterance", so that scenario is "close enough".
To the level of detail that you're focusing on, "sushi" and "the meal" are different.
When you have put one piece of sushi in your mouth, chewed it, and swallowed it, you have eaten sushi. How many other pieces are on your plate is an irrelevant consideration.
In reality, we don't limit ourselves to only saying "having [done X]" meaning that X must now be done. If this is the first time I've heard Metallica play live that doesn't mean I'm not still hearing it. We allow for subdivision. Hearing Metallica play live can be subdivided into smaller experiences which also count as hearing Metallica play live.
However, there are some actions which are inherently bounded, and you cannot subdivided any further. If this is the first time I have punched someone named Robert, then my fist has already connected with Robert. I cannot state this while I'm still in the process of swinging my fist, as that does not constitute a punch (yet), at best it is the intention of a punch.
This is a very contextual process that does not rely on grammatical precision, it's driven by informal convention based on what sounds right.
Therefore, if I slightly change your example:
This is the first time I have eaten 20 pieces of sushi
Then yes, this statement can only be made after eating the 20th piece of sushi.
But "eating sushi" is something that can be divided into smaller steps, it is unrelated to the size of the meal, so that same answer does not apply to your actual question.