The cost of a new display for a 6S Plus is still only a fraction of what a new device would cost you. Even a high quality part would be cheap, compared to a new phone. A new main speaker and charging flex (where the microphone is located) are very cheap parts. The 6S Plus is quite easy to repair, and we do have excellent guides here on iFixit to help you.
Here in Sweden, a refurbished 6S Plus would cost you around 300$, and a brand new one around 400$.
If you do the repair itself, a new display might cost you 90$ and a new speaker and charging flex maybe 25$. If you shop around, there might be even cheaper options, i got these prices from one of the bigger parts retailers (https://www.teknikdelar.se/) in Sweden (Sweden is a quite expensive country mind you).
So yes, in my opinion, it is worth doing these repairs, even at a repair shop. Considering these repairs might cost you around 100$, this is a small price compared to the 3-400$ to buy a new exact same model.
Hope this helps!