What's a good choice of a word or phrase for "subject/target to be analyzed"

"Researchee" is not informal; it is rather a new term with which people are not all quite confortable, as is the case for some new -ee ending terms. The feeling is subjective but people tend to find that such terms are overspecific when used out of expert contexts.

"Analysed subject" wouldn't do as the term "subject does not respond to the function of describing a set of elements with no tight connections but rather to the function of naming a body of more or less tightly interconnected concepts.

"Analysed target" or "the analysis target" appears fine but as you are looking for specificity it is a rather general term and even a new term that seems to be a little overused.

In psychology you can speak of the analysand when you want to refer to the person under analysis, which you can also refer to properly as a subject. In philosophy you have at your disposal "analysandum" when in need to refer to what is to be analysed or clarified (SOED). There is however no indication in the dictionary that would permit to conclude that an extension of the scope of those terms has been recognized or considered acceptable. "Analysee", apparently has made no incursion into any field whatsoever, as I do not find a trace of it on the web or in paper dictionaries. Nevertheless, the verb "analyse" is as good a candidate as any for being complemented by an -ee ending term; many others have more or less recently yielded an -ee ending term (crush/crushee, defend/defendee (rare), acquire/acquiree (1950 (ref.)), honor/honoree (1958 (ref.)),…); in the light of the fact that this suffix contributes to new derivations with ease there is little forbidding you from following the trend that has produced regularly so many -ee ending terms and so introducing a first use of "analysee", but, of course such an initiative might not appear suitable to everyone.

One might consider as possible the use of the -ed ending formation (bless/the blessed, wound/the wounded, age/the aged, damn/the damned, …) but this one can be used much less freely to obtain new terms, and "the analysed", first of all will not on first reading be easily conceived as naming "what is being analysed" and, secondly, there might be much to be said against the creation of this term.

Otherwise there appears to be no possibility of a solution in a single word term. You can say however "the analysed material"; this applies to chemical compounds, written documents, video documents, turnover figures, income, share return, advertising expenditure, … You can also use "the analysed products" (ref.), which will name specifically items that have been manufactured.

The closest single word I can think of, which is more specific than subject, is specimen:

[Merriam-Webster]
1 b : a portion or quantity of material for use in testing, examination, or study
// a urine specimen

A specimen is not a person or general subject, but a piece of organic or inorganic material that is examined scientifically, and very often used as the subject of testing and analysis.

A closely related word is sample:

[Merriam-Webster]
1 : a representative part or a single item from a larger whole or group especially when presented for inspection or shown as evidence of quality : SPECIMEN

As a single word, however, sample can be used in other contexts—as well as being confused with its verb form:

• Can I sample your dessert?
• Can I have a sample of your dessert?
• This is just a sample of the kind of work you will do here.

So, barring context, it might be more easily misunderstood than specimen.

But if used in a phrase in the right context, sample could actually be more appropriate than specimen.

For example, comparing urine specimen, urine sample, soil specimen and soil sample, Google Books Ngram Viewer shows that the sample versions of the phrases are at least twice as common:

In short, if you want just a single word that is the most likely to be understood, and which can apply to any type of thing that is to be analyzed, you might want to use specimen.

If you specify the type of thing to be analyzed, and use it in a phrase, you might want to use sample.

Certain grammatical structures would also recommend one word over the other. Without knowing more about the specific context in which it will be used, I can't make a more informed recommendation.