What is best to use in written language: 'that is' or 'this is' [duplicate]

I am interested to know if in written formal language this is reads better than that is

She goes to the library every Friday, and that is/this is her weekly respite.

Also, I often come across expressions like

That /This is (the reason) why

If I use 'this' rather than 'that' in a written text, does it raise the level of the language, or does it not make any difference?

This and that are demonstrative pronouns or demonstrative adjectives/determiners.

This tends to relate to nouns that are indicated in some way and are close in space or time.

1. This- pronoun and determiner is used with the object/person that is

(i) new [to the experience of the listener.] Introduction: “This [girl] is my sister.”

(ii) the first thing - “This [point] is where we start.”

(iii) presently, ‘for the first time’ – “This [visit] is my first time in Africa.”

(iv) the only thing being presently discussed/experienced - “You have described an interesting idea; this [innovation] could be worth a fortune.”

(v) will be immediately discussed/experienced, - “This [topic] is what we will discuss.” “This [injection] will not hurt.”

(vi) the nearest thing (in space or time) - This [ring] on my finger is expensive.

(vii) the closest thing indicated – “This [watch] that I am pointing at is useless.”

(viii) This cannot be a relative pronoun:

*The man this is wearing a hat is my uncle.

The man that is wearing a hat is my uncle.

This is more immediate than that. It is used to indicate that what follows is of especial interest: “This is a good question" -> This is an interesting question (i) that has not been asked before or (ii) that introduces an aspect that has not been discussed before or (iii) that raises an interesting point. Normally, we would say “That is a good question.”

That tends to relate to nouns that are indicated in some way and are more distant in space or time.

2. That - pronoun and determiner is the object that

(i) has been discussed/experienced, - “I told you about that [mistake] yesterday!”

(ii) is the next and subsequent things, - “This [pin] is rusty, now let’s look at that [points at another pin] one.”

(iii) is a more distant object in space - “That [piece of paper over there] is my shopping list.”

(iv) is a (more) distant object in time. “Ah yes, 1950… that [year] was the year I went to Brazil.” Or

(v) is relative pronoun. “You have described an interesting idea that (i.e. that idea) could be worth a fortune.” (compare with “which”)