Having keyboard shortcuts could be useful to eliminate some clicks.

For the plans offered, could make it clearer that the premium ones include everything in the cheaper plan + more. Also, providing more details what is included in 'advanced features' and what 'full access' means.

As far as bugs, ran a scan with TripleChecker (https://triplechecker.com/s/704482/lira.study). There are a few error pages accessible not from the homepage but for example via https://lira.study/sign-in after you click upgrade, including https://lira.study/guide.

Some of the footer links (privacy policy etc.) are not on the homepage.