The optimal temperatures for making wine is usually stated as between
20 and 30 C; higher can result in stuck fermentation due to yeast
dying off; lower can result in stuck fermentation due to dormancy.
Yeast slow down below 20C, they don't just quit usually. You can
warm it up or rack it to get them going again; racking introduces some
oxygen which the yeast may want. A little dip below 20 C is not a big
deal; it's often done on purpose to preserve fruitiness in whites and
some styles of red.

As to taste at the end of fermentation, that is actually a learned
process. Some reds are gawd awful at the end of fermentation but are
just great a year later. Others are nice the whole time. Most reds
improve with time, if you don't like it just try another 6 months
later. Grape quality and winemaking style affect this to a degree so
there really aren't any hard and fast rules. Just keep good notes on
your likes and dislikes.

Joe