HI Nik,

I'm just east of you in Dorset, and will be in Morocco from March through to the end of May again (4x4 rather than a bike).

1 - no need to pre book the ferries are regular. I tend to check prices online direct with the ferry company and then buy an open return from Carlos in Algeciras.
2 - Morocco is pretty safe from a theft point of view, I've not heard of bikes being stolen but maybe things left in bags etc. Theres a lot of info on here for places to stay with secure parking though. Also iOverlander has some details although after Covid a lot of detail is several years old now.
3 - Camping is absolutely a thing. To me wildcamping is a highlight of any trip. Campsites vary from awful to amazing and price is no indication of quality. Some have closed permanently during covid but the best options are still there. The Park4Night app is good to get recent reviews of sites. Many campsites will also have good cheap rooms available if you dont fancy pitching a tent
4 - A few UK insurers will issue Green card cover for Morocco so check with your current one. Worst case pick up local insurance at the port when you arrive.
5 - Cash is preferable for shops, cafes and restaurants etc. More fuel stations are taking cards now as are the big supermarkets. ATM's are everywhere so its easy to get cash out when needed.
6 - Not my best areas for bike specific routes, but the route from the Dades gorge up towards Imichil is a cracker that will soon be asphalt the whole way so get it done while theres still some dirt left. A lot og good options between Merzouga and Zagora for some desert fun. Although your copy of Chris' book is old it still has a lot of cracking routes in it to start you off. Gara Medour is easily accessible from the main road.

Ramadan is early this year so depending when you arrive you may be during the month of Ramadan. Its not a big problem but I find you need to plan ahead a little more when it comes to shopping.

Main advice would be to get down south on the motorway and spend you time around the High Atlas and the desert with maybe the ride back north up through the Middle Atlas.