I'm not a "big lister" per se but have seen a few birds and hope to see more. A couple friends are young-ish really driven birders with already pretty large lists that are ever growing. We do a lot of trips together - they're just a bit more driven than I am. We mostly bird independently but are happy to use guides where necessary / where it's the right way to do things or where the logistics are so painful that you are happy to have a guide help you sort things out. But mostly, world birding is the same everywhere: Read TR's and talk to friends, get as much GPS as you can not just for birds but for driving directions, trailheads, homestays / park offices / etc, get local contacts, figure out access to sites, learn birdsong, get a local SIM in your phone and go see some birds. Some people use guides more, some less, and there's nothing wrong with any way you want to go about it as long as you're not flagrantly breaking rules / laws everywhere you go.
Personally, I've seen a fairly high percentage of the birds in the neotropics and southern cone by now, and I've only once hired a proper "bird guide" in the W Hemisphere - on my very first trip. Since then I've just had a few days here and there with local trail guides / park rangers as chaperones / boatmen who know where to look for birds / etc, and have probably spent 98% of my days in the new world alone or with my partner or with 1-2 friends. On the flip side, I've seen relatively few birds in Asia, but have hired guides there a few times already, ranging from a straight up bird guide for one or multiple days to many times hiring local guides / fixers who can help sort out access and can take you to territories or areas that you might not be able to find on your own. As in, I can find most birds in the new world on my own bar a few that are in tiny reserves that you have to access with local biologists / rangers / etc. But there was no way in hell I was going to find my own Bulwer's Pheasant so I was happy to pay Calvin Ng in Sabah to drive me up to Trus Madi and take me to his hide. Money well spent!