Ajlittoz’s advice is sound and logical: avoid direct formatting. (He was instrumental in helping me publish my work.)
First set up your computer screen with the Styles pane on the right-hand side of the screen and the Navigator pane on the left-hand side. Don’t use the ribbon menu system - you shouldn’t need it and this will give you more screen space for your writing.
I recommend that you set up/modify three page styles: Opening Pages, New Chapter and Default Page Style. These will allow you to control the content of the Headers, Footers and Page Numbers as used in printed books.
Unless you have a specific reason not to, stick to the standard page size i.e. A4 (portrait) or Letter Size (portrait) if you live in the US or Canada.
Next set up three paragraph styles with the font and character size of your choice and set the line spacing to Double.
Modify Heading 1 as an automatically numbered chapter heading and set the Next Style to be the Default Paragraph Style.
From the Default Paragraph Style create a new style and call it Indent Paragraph Style and set the First Line Indent to 5mm. Leave the Next Style as Indent Paragraph Style,
Modify the Default Paragraph Style and set the Next Style to Indent Paragraph Style.
This will automatically format your work as you write in the accepted “Novel Layout”,
You can find out how to create/modify styles and how they can be linked to work in sequence in the Libreoffice Writer Manual or in my book.
You say that your novel is complicated. I would suggest that if you initially set up my recommendations, you will have acquired enough knowledge to tackle your complications using different style configurations: this is Libreoffice’s great strength. If you run into problems, this forum can usually find an answer.
This is Word-conditioned workflow! Considering the important technical role of Default Paragraph Style in Writer (it is the ancestor of all paragraph styles and it defines default settings for them), never (repeat seventy-seven times) use it for anything else than setting defaults. Notably, don’t use it for your narrative (because you’ll be tempted to customise it and be surprised at the consequences on the other styles). The “standard” style for the narrative is Body Text.
There is a built-in First Line Indent for you to customise.