A full redesign is one idea but if you're wanting to start smaller than that or just learn how to garden the garden you have: I'd pick one thing and learn about it and do it well. Then I'd add another. Then another.
So, if you like your hydrangea then focus on that. There is such a joy in seeing a well loved plant come back to full health.
Some good info here: www.gardenersworld.com/how-to/grow-plants/how-to-grow-hydrangeas/
Check your hydrangea is on the right spot. Then think about what it needs at each point of the year and make yourself a note to do those jobs at the right time.
e.g. they like dappled shade. Does yours get that? if it does = great. If it doesn't, do you want to move it somewhere better? If so, then make a plan to move it at the end of the growing year (around Oct/Nov).
e.g. they like moist soil. Does yours have that? if not, then make a note to water it deeply every other week.
e.g. they like fertile soil. Does yours have that? if not, then grab some hydrangea feed and make a note to feed at least twice during the growing season (maybe once in April and once again in July).
e.g. Does it have a nice shape? If not, make a note to prune it at during spring next year (around Ap/May).
Once you feel you understand your hydrangea, then look to learn about something else. Maybe your roses?
Maybe then focus on problem areas, such as your shady area at the end which could easily become a fernery. Ferns are beautiful and need little care in the right spot.
Piece by peice it'll all get better and you'll learn more and more...