I can comment on Hawaii having done most of it (Kauai, Hawaii and Oahu) between 2017-2020 precovid. If you’re focusing on seeing as many as possible of the extant endemics (and not ABA exotics) then you’re going to need to hire a guide to get to some of the reserves and there was a decline in access even precovid. I can only imagine it’s gotten worse. Avian malaria is really taking a toll on the remaining native bird populations. It’s a really expensive trip per bird, but of course a special experience to see the few remaining survivors of such a rich avifauna.
Oahu: Oahu Elepaio and Amakihi were relatively straightforward on the Lower Aiea Loop Trail. After combing the Kapiolani park trees for White Tern in Waikiki, heading east will get you to Halona Lookout for Red-tailed Tropicbird. Note that Mariana Swiftlet is the only ABA countable exotic which is difficult to connect with on any of the islands, and has become more so in recent years.
Hawaii: Palila is found at the discovery trail and the best public site for forest birding independently is at the Pu'u O'o Trail where you can see I’iwi, Apapane, Omao, Amakihi and Elepaio. Further down as you descend to Hilo is good for Hawaiian Hawk and Nene. You’ll need to contact one of the local guides (I used Jack Jeffrey) for access to Hakalau to get Hawaiian Creeper, ʻAkiapolaʻau and Hawaii Akepa. Due to political conflict with native sovereignty over the construction of a new astronomical lab, the access has been limited in recent years.
There’s great sea watching from shore at Keahole and Keakea including petrels like Hawaiian, Bulwer’s and Juan Fernandez if you time your visit especially between May and September but it’s definitely worth getting on the ocean to get more satisfying views.
Kauai: Independently it is possible without much trouble to get Anianiau, Amakihi and Elepaio at Kok’ee. I was guided for a day by David Kuhn who showed me where to find Akekee (which took two days of searching, the second of which I found the bird at his stakeout independently). Both Akikiki and Puahohi are still extant but not within the searching radius of the average day hiker. You’d have to go on a special expedition to get to where they still can be regularly found. While you’re on the south side of the island, the best spot for Hawaiian Duck was Salt Beach Ponds, which also happened to have a wintering Gray-tailed Tattler at the time of my visit. Of course Kilaluea was fantastic for seabirds, particularly Laysan Albatross.
Let me know if you have any questions.