Hi there,
Improving TTFB is quite tricky as it has 2 sides.
One the WordPress end of things, you can improve TTFB by optimizing how assets are served. You can also try reducing the number of plugins being used.
But this can be futile if the server itself is real slow.
That said, you should test first if the server were your site is hosted is of good quality.
To test this, you can try checking its TTFB on a fresh WordPress installation on a default theme without any plugins installed.
If you get a high TTFB even on a fresh WordPress install then that in itself is a server quality issue.
See general guidelines about it here – https://web.dev/time-to-first-byte
To add: from experience, moving a site to a known good quality dedicated server fixes this issue.