Hello.

Can you please point out any strategic mistakes or generally big mistakes in this game of mine please? (Apart from where the opponent was able to advance into my territory because of misreads) thanks.
I was playing black.

Why K15?
At move 9 I was thinking about R5, but didn't want white to approach from the other side at n3 and after my response at p3 make a 2 space extension. Would that position be ok?

Yes, that's ok for both. I usually play r5 in that position because I like to keep the pressure on white's right side group.

The biggest mistake is that you sort of failed with R8 and Q4 groups. You had a good game going until maybe move . Instead of Q9, you could've played S10, and you would've only had 1 weak group to worry about on that side of the board. But you did not play that, and instead you destroyed that connection. At that point, you sorta entered problems.(Furikawari at S8 was pretty questionable, but I don't think it was a mistake if done with good caution).

So, at move you were in severe trouble, but you had an easy way out, you could've given W something in return for a sente to go back to your bottom group. Saving those would've made the game pretty easy for you. at R14 and then tenuki would've been really good, and you could also have later came back to play S12 to capture W. But for whatever reason, you did not do that, and now you're starting to owe moves all around the board. With left side having yet another weak group, you're in a lot of trouble.

And the game ends with . After that there are still moves but for all intents and purposes the game is over, you're just fighting over the exact amount of points you lose with.

The S13 alone maybe didn't lose you the game entirely, but you really can't afford to make such bad shape. You could've used R13 to build a nice top side in sente, but instead of that you sorta got completely destroyed for a very weird and unintuitive shape. Trying to connect under is not a reason good enough to make a shape like that, it would be better to die with R8 group than to end with such a shape. Before that, if you were willing to sacrifice one of the three weak groups on the bottom side of the board, and focused on building the top, you could've made it at least a pretty close game. White was ahead, and I can't promise you would've won, but following that path you at least could've waited for W to make mistakes in direction or reading.(My poor points estimate would be that two StlenVlr's playing would result in W+10 in such a game)

Thanks for your comments.

Was the idea to reduce with R6 good? And what could I locally or globally have played instead of S8?
Was D18 a good idea?