He was 80- years- old and was suffering many health problems due to hits great age.

(1) We do not hyphenate "80 years old" here. We only hyphenate it when it is an adjective preceding a noun ("the 80-year-old tree") or when it is a noun itself ("he is an 80-year-old (person)").
(2) Your uncle is a male, so we use "his" instead of "its".

One year before, atago, around the same period of the year, one of my aunts had passed away as well.

(1) When using the present as the reference point when talking about past events, "ago" sounds more natural to me than "before".
(2) "Around the same period" sounds more natural to me. There is also no need to specify "of the year", as you have already established that this occurred a year ago.

She was about the same age as my uncle, maybe 78- years- old.

See my comment on "He was 80 years old...".

A cancer had been diagnosed at an advanced statge, and I imagine (that) she could hardly endure the harsh treatment because of her great age.

(1) "Stage" is typically used when talking about cancer. We say "stage 1 cancer", "stage 2 cancer", etc.
(2) You may omit the "that".
(3) Instead of "great age", you can also say "old age". It will introduce some variety in the vocabulary.

I’m 47- years- old, so these losses are not the first ones among my relatives.

Unfortunately, experiencing (the) deaths of beloved ones is a big part of everyone's life.

(1) "Beloved ones" is plural, so the plural "deaths" is used.
(2) "Everyone" is not used as an adjective.

But for the recent losses, my uncle and aunt, the context was different and I sadly realised we lost much more thatn I had imagined.

The generation before me, my parents and their siblings, haves reached an age where Mother Nature calls you back.

"Generation" is singular, so the singular "has" is used.

Not to mentionBesides our broken hearts, my siblings and I are also expecting a huge milestone in our lives — eventually becoming independent adults.

This usage of "not to mention" is a little awkward. For now, I have replaced it with another word that works in this context. However, if you can provide the sentence in French, perhaps I can find a more suitable word to use here.

But it’s much more than just about us, as they are more than elderly people who are dying.

I assume "they" refers to your relatives. I would suggest adding "who are", to make it clear that "elderly people" is the noun. Otherwise, the event of "elderly people dying" may be interpreted to be the noun instead.

I remember my mother telling us about the revolution ofthat was the first landline phone in her village when she was a child, as well as so many other stories — the installation of running water, the moveing of the toilet from the garden to the house, the use of a washing machine instead of a washboard, the central heating instead of a hot brick under the bed, and so on.

(1) "Revolution of the first landline phone" may mean that the landline phone was the thing that underwent a revolution, rather than the village.
(2) "The moving of" sounds more natural than "the move of".

Besides their personal life experiences of life, they are also living witnesses to our collective past.

"Life experiences" is a common term. Alternatively, you may also say "personal experiences in life".

He was 80- years- old and was suffering many health problems due to its greathis old age.

One year before, at the same periodtime of the year, one of my aunts had passed away as well.

She was about the same age as my uncle, maybe 78- years- old.

A cancerShe had been diagnosed atwith an advanced statege cancer, and I imagine that she could hardly endure the harsh treatment because of her greatold age.

I’m 47- years- old, so these losses are not the first ones among my relatives.

The generation before me, my parents and their siblings, haves reached an age where Mother Nature calls you back.

But it’s much more than just about us, as theyre are more than elderly people dying.