Jhana

[auto]

..

Have you considered that the animitta(signless) simply referrers to rupa what is not an object of sensual pleasure anymore?

Someone who desists from sensual perception,
Virato kāmasaññāya,
has moved past the fetter of form,
rūpasaṁyojanātigo;

List of objects of sensual pleasure,

There are many objects of sensual desire:
Khettaṁ vatthuṁ hiraññaṁ vā,
fields, lands, and gold; cattle and horses;
gavāssaṁ dāsaporisaṁ;
slaves and servants; women and relatives.
Thiyo bandhū puthu kāme,
When a man lusts over these,
yo naro anugijjhati.

Last excerpt is a reference to the aggregates, all aggregates are mental formations, including the Rupa aggregate.
Regards

you consider sn suttas early and credible source of buddha's teachings so,
sankhara(mental formations) is 4th khandha,

And why do you call them choices?
Kiñca, bhikkhave, saṅkhāre vadetha?
Choices produce conditioned phenomena; that’s why they’re called ‘choices’. Good luck translating this.
Saṅkhatamabhisaṅkharontīti kho, bhikkhave, tasmā ‘saṅkhārā’ti vuccati.
And what are the conditioned phenomena that they produce?
Kiñca saṅkhatamabhisaṅkharonti?
Form is a conditioned phenomenon; choices are what make it into form. Feeling is a conditioned phenomenon; choices are what make it into feeling. Perception is a conditioned phenomenon; choices are what make it into perception. Choices are conditioned phenomena; choices are what make them into choices. Consciousness is a conditioned phenomenon; choices are what make it into consciousness.

*** translators note = Choices produce conditioned phenomena; that’s why they’re called ‘choices’. Good luck translating this.