Age of Empires: Definitive Edition: Yamato, Empire of the Rising Sun: Hardest: 2) Island Hopping

Island hopping is the second scenario in this campaign, and I think it's a great level as it highlights some of the strengths and weaknesses of different units. It's also a very tricky and cheeky level with a few traps. The goal in this scenario is to capture 6 artifacts that are scattered on different islands in the region. You don't have to route the enemy or relocate the artifacts, simply capturing them is enough.
You begin with:
1x: catapult, ballista, light transport, heavy transport
2x: war galley, phalangite
3x: composite bowmen
The first task, as given in the hints, is to find the source of the shikoku galleys that you'll encounter quickly in the scenario. If you fail to find the shikoku dock, they'll overwhelm you with galleys and just pick your units off. (Which is good as this creates a sense of urgency) I found it easiest to follow the ships like a trail of breadcrumbs, which eventually leads you to the location of the docks. Even though there are 4 docks in total, only one produces ships (the Imperial age looking dock). You need to dump your land units somewhere close on an island so they can assist in taking out the galleys as your own galleys will be too slow, while avoiding casualties from Red units on the islands. The composite bowmen and the ballista will help a lot in that regard while using the catapult should be avoided as its area damage can destroy your own units.
Eventually, the main dock is found to be in the western corner of the map. The amount of time it takes to destroy this dock will strongly influence whether you pass or fail this mission as you will take casualties trying to eliminate this dock.
Once you've destroyed this dock, you can relax and take stock of what is left.
The catapult is the strongest unit in this scenario. With a long range, area damage and massive line of sight, it can scout, deal massive damage and fire over cliffs and across the sea without fear of being attacked by other units. The ballista and composite bowmen are also good here, but don't have the same range or damage, while the Phalagites are useful in soaking up damage or acting as decoys for your ranged units.
Once the dock is destroyed, use your leftover war galleys to explore the map, taking stock of potential enemies, the location of artifacts and the location of the islands. You can harass units with them, but they'll probably be taken out by bowmen. If you don't have galleys, use the transport ships or if on an island, use the composite bowmen, the ballista or the catapults' line of sight to reveal other potential islands or enemies.
Try to use the catapult's attack ground option and range to be able to pick off bowmen, priest and other units, as simply attacking them normally will cause them to move around and dodge your attacks. The composite bowmen/ballista will be able to snipe units as well, but not as effectively as the catapult.
With exploring, you'll find the locations of the 6 artifacts (in no particular order):
1) On the north corner island. Guarded by 3 composite bowmen
2) On the southeast center island. Guarded by 3 lions
3) On the central southwest island. Guarded by 2-3 composite bowmen
4) In the western corner, on the shoals. Guarded by bowmen
5) In the western corner, close to the granary and 2 axemen. Loosely guarded by centurions and bowmen.
6) In the western corner, close to the docks. Guarded by bowmen.
Artifact #2 is easy, as a composite bowman or a centurion on low health can mop 3 normal lions with barely a scratch.
Artifact #3 will take some team effort to corner the Comp. Bowmen, but your ballista or 3 Comp bowmen should be able to take them down with no casualties. The catapult may be able to snipe them from other islands though. This will be tough with few units.
Artifact #1, the island is isolated so it's difficult to snipe anything from afar, but the island is small enough that 2 centurions or centurions and a partner should be able to win.
Artifacts #4 and #6 are loosely guarded by numerous bowmen. Your ballista, catapult, war galleys and archers all outrange them, so you should be able to snipe them or kite them from afar. The centurions will also destroy them if they can get close enough.
Artifact #5 has the potential to be the trickiest. There is a contingent of centurions and various archers to the east of this artifact, that can be provoked should you go too far. Best to kill the axemen guarding it and leave. A few of them can be sniped with the ballista/catapult from other islands but hit and run tactics (with or without the transport ships) will also work here as well.
You win once all 6 artifacts are under your control.