It depends on what expectation you have of the langugaes learned. To me, it would be at least C1, as this is the comparatively advanced level where you will feel comfortable with the use of the language in general. You can read analytical essays with ease, and carry on meaningful conversations with native speakers. And that is the level that all serious language learners want to achieve. It makes you feel that you now partially "own" the language.
I am not yet a polyglot, but I had the experience of learning several languages simultaneously in college. At a time when my English was average, I shifted my focus onto Japanese and French because I felt bored learning English all that time and wanted to have something new. It was fun at first, but got me nowhere.
So I would say although starting to learn a new language when you are not advanced in the previous one might add novelty to the whole experience, it bears the risk of achieving little in the end.
- it seems to be a good idea not to drop Mandarin. At least none of the polyglots I know ever dropped anything eh is good at today! And of course, there is every reason to believe aegis' experience.
- though I don't think that starting former A2 again will take the same effort. THere is a number of former A1/A2 langauges which i forgot... but I can't imagine myself spending the same time trying to renew it. I was unexperienced, I spend much time on figuring out things which are obvious now.
- THe premise that learning language A must be stopped when you are focusing on the language B seems to be unnecessary. I see no reason why shoud you drop Mandarin if you want to learn French.
Learning two languages at once is norm for many people. May be a pause for a month doesn't count.. But no, dropping Mandarin is a bad idea, most likely. Still, French is EASY for an English speaker compared to Mandarin. Why not both?
- if you only can invest limited time/efforts in French, then at least assure that you are not doing the same thing with your French all over again. Or it will be 'maintaining', without real progress.
- what is really interesting... is what happens to such people, multi-lingual since youth when they are 60? This is an important detail, that the guys I speak about all are young. They experienced immercion quite recently... but may be when you are 40 or 50 maintaining becomes one of primary concerns.
So it would be instructve to know more about aegis' early hiustory of learnign language. If Aegis started from, say, Spanish - then how his Spanish evolved (devolved:)) over years?
As I'm sure you've already suspected, different polyglots make different decisions about when to do this. Remember that there is a wide variety of language learners who call themselves polyglots. Their credentials range from two languages with a basic level, to over a dozen languages with advanced levels. So it's a good idea to check a polyglot's credentials before following their methods.
Imo, most serious polyglots wait until they've reached at least an intermediate level before they add new languages. Although I've made mistakes about this in the past, I recommend reaching at least B2 before starting another, and it's better yet to go all the way to your target level, or as you say become "satisfied", before you move on. The reason is that it's much easier to maintain a language in which you've reached a high level than a low one.
If you move on when you are still A1/A2, you will need to spend as much time maintaining it as you did learning it, so it makes no sense to maintain it. When you finally decide to pick up that language again, you will need to spend about the same amount of time you did the originally to regain your level. So if your goal is to learn several languages to a high level, don't start one unles you have the time to take it all the way to at least B1, preferably higher.