There are several potential factors contributing to dieback, and at this stage of tree decline, unfortunately it would be more practical to replace the tree. Ornamental (flowering) cherries are vulnerable to an array of pests and diseases, though mostly when stress or injured, and they are also somewhat short-lived (perhaps 20-30 years overall). The linked page provides more information about the various ailments typical for cherries to contract, though some are more serious than others. For your tree, it's hard to say what the primary or original issues are, but Brown Rot, borer (insect) or canker (fungus) infiltration in certain branches, and being planted too deeply (and/or being over-mulched) are likely factors. Unfortunately, the fact that it flowered isn't necessarily a good sign; as trees age or decline, they tend to shift growth priorities to reproduction rather than leaf or branch growth, since blooming in order to produce seeds is a last-ditch effort to perpetuate the species since they can't save themselves, so to speak. Both healthy and dying trees can flower well, so it can be an unreliable trait for diagnosis.

Regarding planting depth, if you want to keep and rehab this tree (or when you plant another), try to make sure the root flare is located just at the soil surface and not buried several inches below it (or below mulch, as it should remain uncovered). The root flare is the junction on the trunk base where major roots attach. When buried, they risk girdling the tree, affecting sap flow and, in essence, "strangling" it; roots buried too deeply also receive less oxygen from the surface, which can stress or kill them, particularly in conditions of compacted or wet soil. Mulch is valuable to use, especially when a tree sits amid lawn so mowers don't get too close to the bark, but it should be used in a flat disc layer around 3 inches deep or less, and kept off of the trunk base and root flare itself.

There are a couple branch stubs visible that had been pruned back which are not cut in the proper location, so that the dying/dead wood in those stubs may be harboring wood decay that could spread to other areas (if the tree can't block it off internally). If you keep this tree, we suggest having it assessed by a certified arborist (especially if they are familiar with flowering cherries) so they can determine where to cut to remove dead or weak wood and how to fix some of its remaining branching structure, which is fairly poor at this point. They can also look for (and sometimes correct) girdling roots and check planting depth, though the latter is hard to correct for an established tree.

Arborists sometimes recommend fertilization for struggling trees, but rarely is that warranted unless a laboratory soil test demonstrates that certain nutrients are deficient in the surrounding soil. While occasionally fertilization with certain formulations of readily-accessible nutrients might generate results of new growth, there is limited scientific study supporting their use; you can ask an arborist about their experiences and success rate using such a treatment if they recommend one. Generally, though, sick plants should not be fertilized unless a nutrient deficiency in the foliage is the only problem, which is not the case here. Otherwise, adding extra nutrients may worsen the tree's stress, or at the very least, its root system won't be able to make use of the added nutrients.

Similarly, an insecticide or fungicide treatment is not warranted, since they cannot reverse existing damage and cannot cure established disease. While there are limited occasions where their use as a preventative can be helpful, we don't think the current state of this tree would benefit from their use.

If you are wary of an arborist making recommendations for unnecessary treatments, you could look for a "consulting" arborist that isn't affiliated with or employed by a tree care company, as they may have less motivation to suggest treatments that stand little chance of working. To be fair to arborists, they cannot always narrow-down a diagnosis to an exact cause, especially if it began years ago (as tree decline often does), and they cannot treat every ailment, even if a condition is worth intervening to address. Some arborists have more diagnostic experience than others, so you can ask about their familiarity with cherry problems if it helps to select someone to hire. If you decide to replace this tree, though, you can skip the arborist consult if you wish.

Miri