Grammatically, the sentence above is correct? I have hesitations for e.g. "vor rund eineinhalb Jahr" or "vor rund eineinhalbem Jahr", and whether "seitdem" is in the right position?
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The sentence is semantically wrong.
Let's split it up:
This is a point in time. You cannot insert something with seitdem as this word describe a range of time, or more precisely since a time in the past until now. But you can use, for instance, als becaus this word denotes again a specific point in time.
If you want to use seitdem, you first must make up a time range:
Here you refer with "In den letzten Jahren" to a range of time and then again you can insert something with seitdem.
I assume you want to say that somebody changed in the last one and a half year of your relationship? Choose this translation:
Person works here, but Mensch is more common:
is almost correct (Jahren instead of Jahr), but it describes a setting you most likely do not mean:
(You could also write it like this then: Vor rund eineinhalb Jahren – seitdem habe ich dich kennengelernt – warst du eine andere Person (für mich).) This sentence might work e.g. in a mystery story.