Is Buddhism a reduced version of Samkhya philosophy? I have read a couple of articles written mostly in passing by western translators on the influence of Sage Kapila (i.e Samkhya) on the Buddha, but I'm yet to find a detailed inquiry on a similarity of these systems.
We read that Alara Kalama one of the teachers of the Gautama Buddha was a follower of the ancient Samkhya philosophy. it is also written that the Bodhisattva Gautama eventually equaled his teacher Alara, and went on inquiring further until he was fully awakened, Buddha. And after awakening, the first person he wished to teach was his former teacher Alara.
I'm only mentioning that above to highlight why I venture to inquire in this direction, my question is mainly on the similarity of the two systems /thought. I will list some:

For both systems, the main goal is to 'End Suffering' by removing ignorance.
Contrary to most religious thought in India, both systems, although they accept heavenly beings, they reject the idea of a creator God. The gods they accept  are just like humans bound within the cycle of suffering. Samkhya satirically declares 'No one no-where Is happy'. The idea of 'Brahman' is also rejected.
Again both say that there is no-self on forms, sensations, perceptions, and all mental activities including the mind's reason [Buddhi].

Now, there is a 'Self' in the Samkhya system, but on critical reading, it's beyond any of the non-self listed in the Buddist thought. It's not a self that transmigrates and pile-up karma it's unbound ever free self. What is Bound and what is freed is the mind, not the Self.
Similarly, there are primordial matters in the Samkhya system. But Buddhism doesn't discuss or is not interested at the beginning of the matter, etc...
There is a lot to list here but in short, when one compares the two systems it seems that Buddhism is a reduced version of Samkhya with some of the ideas removed as non-essential. Was the Buddha a reformer?
Many Thanks
According to the wikipedia page on Samkhya:

The earliest surviving authoritative text on classical Samkhya
philosophy is the Samkhya Karika (c. 200 CE or 350–450 CE) of
Īśvarakṛṣṇa. There were probably other texts in early centuries CE,
however none of them are available today. Iśvarakṛṣṇa in his Kārikā
describes a succession of the disciples from Kapila, through Āsuri and
Pañcaśikha to himself.

So, Samkhya philosophy as known today, came almost 700 years after the Buddha (c. 563-483 BCE) based on the date of Samkhya Karika. But as stated above, it is attributed by Iśvarakṛṣṇa to Sage Kapila, who obviously predated the Buddha.
It is possible that the Samkhya Karika had experienced influence from Buddhism.
What about Alara Kalama?
Apparently, the only source stating Alara Kalama as following Samkhya is the Buddhacarita, composed in the second century CE by Aśvaghoṣa. This too was written at least 500 years after the Buddha.
There is no mention of the Samkhya philosophy by name in the Pali Canon. The Pali Canon also never explicitly attributed any teachings to Sage Kapila. The only mention of Kapila is in the Jataka Tales, dated at least a century after the Buddha.
In any case, whatever doctrine or teachings that was taught by Alara Kalama to the Buddha, did not bring him enlightenment, as we see in MN 26:

&quot;In this way did Alara Kalama, my teacher, place me, his pupil, on the
same level with himself and pay me great honor. But the thought
occurred to me, 'This Dhamma leads not to disenchantment, to
dispassion, to cessation, to stilling, to direct knowledge, to
Awakening, nor to Unbinding, but only to reappearance in the dimension
of nothingness.' So, dissatisfied with that Dhamma, I left.

So, the question is - did the Buddha copy and reduce Samkhya, or did the Samkhya Karika copy the teachings of the Buddha and tried to reconcile Buddhism with the teachings of the Vedas and the Upanishads?
Based on the dates of the writings, I would say the latter is more likely. So, you've got it in reverse.
But did the Buddha copy Sage Kapila? Well, this is impossible to say, because we don't have any authenticated pre-Buddhist writings of Sage Kapila remaining, in which he taught Samkhya.

Based on the following statements from the Samkhya Karika, the definition of Spirit (Purusha) is in violation of sabbe dhamma anatta (all phenomena is not self). There is no single Self or Spirit that is the pure witness or controller in Buddhism.

The Spirit exists because (a) the aggregate is for another's sake; (b)
of the absence of three guṇas and other properties; (c) there must be
some controller; (d) there must be some experiencer; and (e) of the
tendency of activities towards final beatitude. - Samkhya Karika
17
The multiplicity of the Spirit is verily established (1) from the
individual allotment of birth, death and the instruments, (2) from the
non-simultaneity of activities, and (3) from the diverse modifications
due to the three guṇas. - Samkhya Karika 18
And from that contrast it is established that the Spirit is the pure
witness. He is solitary, neutral, spectator, and non-agent. - Samkhya
Karika 19