Brown spots or leaf tips are usually an indication of irregular or insufficient watering. Could also be an indication of sunburn but that is far less likely with a tree established in place for some time although the two issues can work in tandem.

Dieback of smaller stems is also common with JM's and sometimes these can progress to larger branches. There can also be pathological reasons for dieback as well but that is not possible to tell from the photos provided.

if it were mine... i would give it a deep thorough watering.. clean up all the dead stuff ... and ignore it .. other than admiring my brilliant pruning job.. lol ...

its a tree... it may be dying.. it might take a decade to die ... and the only thing to speed that up might be your tolerance to look at it ...

its probably been there decades ... taking care of itself .. else how did it get that old ... and it probably didnt need any help from you or the prior owner... so what need be done now ...

if it dies.. it dies.. its the circle of life ...cue elton ....

ken

he look so young: https://www.imdb.com/video/vi1988540953

Almost all of my JMs in sun do that. I finally got rid of (or moved into full shade) any of the ones that I had in sun. I also rarely buy JMs that are desirable mostly for fall color as the leaves are so crisped by the time that fall approaches that I don't get fall color. The ones that work well in sun here are 'Seriyu', 'Tobioshi' and 'Red Dragon', among a few others. I went to Seattle a few years ago in the summer and saw how gorgeous the JMs were there and realized that I do not live in an ideal place to grow them.