I am familiar with たり when used for multiple actions. In the following sentence:
人は どんな環境にも慣れるんだ
と…思ったりする
(The man is thinking to himself and this is his conversation with himself)
How the use of たり affects the meaning and sense of 思ったりする when it is not used for multiple actions?
You are of course right about 「～たり（～たり）する」being used in its multiple ～たり form to list several possible actions, but it does not HAVE to list multiple possible actions in order to be used to represent more than one action.
When only a single action is mentioned, there are two ways to look at it. Either there is another unmentioned action that might happen, or there is more than one possible object of the single action mentioned.
Consider:
週末にはROUND ONEで遊んだりする。
It's pretty clear that the person is saying that on the weekends they play at Round One. Either they also do other things beside play on the weekend, or else they also play elsewhere besides at Round One. Because this ～たりする form is so open-ended, either way you want to think about it is valid, until context provides more clues.
So going back to your example
人はどんな環境にも慣れるんだ
と…思ったりする。
...In this case, since it seems unlikely that the speaker would do some other action than "think", he must mean that he also thinks OTHER things, besides the thought that he chose to mention.
therefor I would translate this as:
People will get used to any kind of environment... I sometimes think.
The reason I choose to use the word "sometimes" is that it is the best English choice to represent the idea that it is only ONE of the things this person might think, thus preserving the sense of ～たりする.
It would have been nice to have some context, like what was said before (and after) this sentence. Although it wouldn't really change the meaning, we would be able to give you a more accurate translation.
As for the question, たり has the same meaning whatever the situation it's used in, it's used to imply that there are other actions besides the one stated, which is why adding context would help us better give you an accurate translation.
人は どんな環境にも慣れるんだと…思ったりする : I think (among other things)... that people get used to any environment
Each time I heard this expression, I would probably have translated it as "[Maybe I'm wrong but] sometimes I think that...".
I was going to start my own question similar to this one because I have just encountered this in a piece of news on the NHK WEB EASY site:
新しいコロナウイルスのワクチンを受けたあと安心して、マスクをしなくなったりするお年寄りがいます。とても困っているとSNSに書く人が増えています。
(Not sure if they keep all the news available for long, but the link will work for now at least)
Having first checked some other explanations (1, 2, 3, 4), I found this answer and decided to post some links here and give the example above of what can also be considered a "concise use" of たり where you give just one example action (as opposed to at least two)
In this case, as I interpret it, the single action of マスクをしなくなったりする implies that there are other actions that senior citizens might be doing that troubles other people (I would assume not washing their hands, not practicing social distancing, etc.), and we actually see one of this in the examples in the subsequent paragraphs:
例えば「父がワクチンを2回受けたあとマスクをしないで出かける」とか「店に来た人が手の消毒をしない」などです。
マスクをしていないお年寄りが「ワクチンを2回受けたから大丈夫」と近くから話してきました。
I would also like to point out that if you follow those links I gave above, many people are quite strict in their views on this grammar, while some provide examples of grammar books that acknowledge this increasing flexibility as early as in 1970s.
And so, now we even encounter it in official news, albeit for foreigners. Or maybe I should say "even though" because this is where I would partially agree with those who try to stick to the rules: since this source is aimed at foreigners, with what is called やさしい日本語, I would personally expect them to use regular beginner/elementary grammar constructions, i.e., ～たり～たりしない in this case. If they are doing it consciously, even in materials for foreigners with only basic Japanese skills, it might be a proof that such flexible use of たり is actually quite common
