Optimisation Guidance for WordPress Site on Lightspeed Server

We are looking for someone to help us squeeze the best performance possible from our WordPress website, particularly for mobile user. The website is www.orchidinfo.co.uk.

Using Page Speed Insights we are currently getting ~82 on desktop but only around ~22 on mobile. We would like to see both improved. Desktop should regularly hit 90 or more and mobile should hit at least 50.

We can ready the issues highlighted on PSI, Pingdom etc. What we are looking for is specific guidance and suggestions on how we can fix/improve them on our actual site.

We have been thinking about AMP but the AMP pages look bland if you just install the AMP plug in.

The website has been built by our web designer and he needs to be advised/shown what needs to be changed, in detail, so that we can be sure that we don't disable any of the improvements with future updates.

The WordPress Website running on a LightSpeed Server and uses the associated LightSpeed PlugIn to auto optimise graphics etc. We want to keep LightSpeed (which might need better configuration) as a part of this solution, unless there is an overriding reason for changing.
See https://docs.litespeedtech.com/lscache/lscwp/

If necessary we can create a staging area for testing. However, some registered plugin won't function on a different url.