The safe option if you're unsure about the pronouns is to ask about pronouns or to use the name. Most people consider it ok to use gender neutral pronouns if you don't know the person yet, i prefer to avoid pronouns entirely in such situations and use descriptors like "that person over there" or "the OP of that thread" or whatever fits in that case - it's a bit more stilted, pronouns are a useful tool in language, but sometimes it's just better to avoid them. The reason i'm not a fan of default they / them is partially because gender neutral pronouns are used for deniable misgendering by terfs, partially it's because my first language doesn't have widely accepted gender neutral pronouns and any usage of neopronouns like dey /dem is highly specific and individual, partially it's because even as a nonbinary transfem, it feels kinda shit to me personally to be adressed in a gender neutral way. You can't just assume everyone's fine with neutral terms. My gender isn't neutral just because it doesn't neatly fit into the box society has constructed for femininity. Being nonbinary doesn't make you androgynous by default, a sizeable number of trans people aren't nonbinary and a sizeable number of nonbinary people who do use other pronouns than he / him or she / her do not go with they / them, either.
ofc it's always ok to use they / them when you're talking about groups and use it as a plural, or when you're talking about unspecified people instead of certain individuals. But if you're talking about a distinct person and can't ask them their pronouns, pronoun avoidance works better than just using they / them as a hotfix.
"Clearly going for one or the other" doesn't always work that well. The default should be to always ask for pronouns, and that should be normalized a lot more. I know from personal experience how validating it feels to be gendered correctly on a guess, i love when that happens, but i know so many people where appearance and pronouns do not match, especially when it comes to neopronouns. How do you guess a transmasc person is using en / en instead of he/him or they / them? Or that somebody uses plural they because they have DID? Or that the person in front of you who "looks like using he / him" pronouns is a boymoding early transition trans woman already going by she / her in the circle you're in? Or if the lesbian over there uses she / her, xier / xiem or is one of the elusive he / him lesbians? Or if that high femme that you've automatically assumed to 100% be a cis woman is actually a nonbinary transfem who prefers dey / dem over she / her?
These aren't hypotheticals either, these are all people you'd run into at any trans meetup in my local queer community. That's a normal saturday night for me. Sure, we also have a ton of femme presenting she / her catgirls and masc presenting he / him skater boys, but not all of them have perfect passing, either.