I'd use a hyphen.
You could use a semicolon after tender.
You could use a semicolon after tender.
I not infrequently use semi-colons regardless of the formality of the writing or of what I'm writing about.
When I first read it, I thought there were three ways this could be written.
1. That meat was so tender! It must have been slow-cooked.
2. That meat was so tender; it must have been slow-cooked.
3. That meat was so tender [that] it must have been slow-cooked.
There is no difference in meaning between 1 and 2, but 3 is a bit different. Can you work out why?
Honestly, I cannot tell the difference between 2 and 3 !
Honestly, I cannot tell the difference between 2 and 3!
After saying all that, it is quite possible that alpacinoutd intended there to be two separate sentences. If this is the case, the word so takes on a different role (as a simple intensifier) and correspondingly has a different intonation. Without having access to the pronunciation, we cannot be sure which was the intention.
alpacinoutd—could you tell us a little more about what you mean to say? How exactly are you trying to use so?
I see. In that case, you were right to use two separate sentences. But do you see how emsr2d2's suggestion of using an exclamation mark at the end of the first sentence (variation 1 from post #8) helps the reader to know that that is what you meant?
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