The rate of starting to completing the checkout was ≈35%:

Some of the reasons were:
- website timeouts due to exceeded RAM limit on the shared hosting
- different PHP errors
- the checkout page was using the new Woocommerce checkout block which wasn’t compatible with GLS shipping and BoxNow delivery plugins
After I made the following changes:
- migrated the website to a VPS hosting
- fixed bunch of PHP errors by applying theme and plugins updates, clearing transients
- rebuilt the checkout using the traditional Woocommerce shortcode (to ensure compatibility with the GLS shipping and BoxNow delivery plugins)
- changed the “Place order” button color from red to a shade of green
- removed “Subscribe to our newsletter” section from the checkout page
- added FAQ section to the the checkout page
- optimized the website for performance
- enabled cash on delivery payment method
the rate for the same period (20 days) became ≈48%:

This is an increase by ≈37% ( (48-35)/35 ≈ 0,3714 ).
Unfortunately, the client hadn’t had a budget to solve all tech issues so the final result could be better. But still, it’s an improvement.