There are two potential contexts for this:
There is no alternative term in game mechanics. Period. End of story. Humanoid is a specifically defined creature type in D&D 5e (and 4e, 3e, 3.5e, and both first and second edition Pathfinder). In particular, it refers (just like the term in real life) to a standard tetrapod-derived body pattern (four limbs, one head, optional tail) with a bipedal stance, and has a number of associated ‘default’ statistics used in creating NPCs who have this creature type. There are a handful of things in the actual game mechanics that care about uniquely identifying this type of creature (though far fewer than in some other games), with the most notable example being the spell Hold Person (which only works on humanoids).
Do not go around changing these terms in your games in the context of game mechanics. Aside from probably confusing your players (‘I thought the villagers said they were attacked by a puddle, what the hell is this gelatinous cube doing here?’), you open yourself up to long complicated arguments resulting from misunderstandings of creature types (and I have seen plenty of those with the existing creature types).
However, this does not mean that this is the term used in-universe by most people. In-fact, while creature-type names may be used in-universe, chances are they are both language-specific and mostly relegated to discussion among adventurers or scholars, just like most normal people IRL talk about monkeys and apes instead of simians, or lizards and snakes instead of squamates, or earthworms and leeches instead of annelids.
Rather importantly, in-universe you can use whatever term makes sense in the context of the discussion, which brings us to the second context this question could be taken in.
This gets more into a writing or story-telling question than one of game mechanics, but I’d argue it’s still on-topic here because storytelling is a core part of the game and it’s most of where you’re going to run into arguments about human-centrism.
For your first two examples, the proper way to phrase the questions is in terms of INT scores and contribution to local society. More specifically, in both cases what both examples care about is creatures with an INT >= 3 that actively contribute to society by their own choice. The simplest solution is to use the term almost all of your players would probably use here, which is ‘people’, or possibly ‘races’ if you care less about individuals and more about species. In other words, both are likely to be phrased exactly like they would be in real life.
For more intellectual discussions that aren’t trying to put down the individuals being talked about, ‘sapients’ (used to refer to individuals with sentience (language, tool-use, and self-awareness) and the ability to reason about the future, though this arguably includes a lot of INT 2 creatures also if you go by real life) or ‘sophunts’ (a term originally from sci-fi referring to sapient creatures that have a level of intelligence at least equivalent to humans). Both are more technical terms in real life, but there is no reason to believe that such terms would not exist (translated of course) in the languages in the game universe.
Where it gets interesting is cases like your beholder (or dragons, or other high INT creatures who do not have humanoid bodies). Here, I’d argue that ‘humanoid’ is out of place not because of the human-centrism of the term, but because it’s not insulting enough. Unless the character being portrayed has a serious lack of creativity or is known for a particularly clinical style of speech, the use of the term ‘humanoid’ would stick out to me as being strange because it’s too bland. You’re trying to portray bigotry and racism here, such characters don’t pull their punches. Terms like ‘monkeys’ or ‘apes’ are the first alternatives that come to mind. Perhaps ‘fleshbags’ if it’s a being that either has no corporeal form or has a synthetic body. Maybe ‘warm-bloods’ if it’s a naturally cold-blooded creature. Perhaps ‘trogoldutes’ (literally ‘cavemen’ in the real world, though they’re a particularly reviled species known for their stupidity and horrendous smell in the TTRPG settings in which they exist) if it’s supposed to be a truly generic insult. If they just care about intelligence, power, or social standing, then any of ‘peasants’, ‘plebians’ (the ancient Roman equivalent of ‘peasants’, usually used today to insult someone’s intelligence), ‘boors’ (derived from a Germanic root meaning ‘farmer’ or ‘peasant’, used today to refer to people who are uncivilized or socially inept), ‘neanderthals’, or something similar would work.
The important thing to remember here is that ‘Common’ is not ‘English’. Even things being said in Common are being heard through the lens of translation, so what matters is getting meaning across. Just because there wasn’t some ancient Greek warrior named Ἀχιλλεύς doesn’t mean that there isn’t an idiom that has exactly the same meaning as ‘Achilles heel’, but it would be stupid to spend time explaining the in-universe idiom to your players and then hope they remember it, so it just makes more sense to say ‘Achilles heel’ instead and be done with it. Similarly, you don’t need some ‘fancy’ politically correct word, you just need one that gets the meaning across sensibly, which usually translates to speaking as you would in real life.