My first prints from my GH5 are a disaster! Where am I going wrong?

This is something I learned the hard way - I will try to explain it simply for you.

Assuming youve made all the adjustments you want (including pre export sharpening) - , After you have arranged all the photos into the photobook - see if you can find the exact print dimensions of each image (many software allow you to see the properties of each image). Then go back into lightroom and re-export every image at the exact dimensions that are in the photo book. Sharpen the image also during scaling/export (light room sharpen for print normal or high) or sharpen post export using another software. This is time consuming yes but it ensure your images are not altered by the photobook software and maintains the sharpness you want.

Do not trust any photobook software to manage your images except to arrange them on the page. Do not scale or make any manipulations to the image within the photobook software - this is very bad (except during the initial arrangement stage). If you need to make adjustments - do it again in lightroom with the original image and re-export with the right dimensions again.

e.g. if you realise you need to crop a photo in the book, go back to lightroom and do the crop with the full res image and re-export again.

Yes this is time consuming but this ensures your images look exactly the way you wanted them to.

Also check what DPI the printer uses. Its important you match the DPI when exporting your images. For print this is usually 300dpi but sometimes higher.