I watched and researched numerous ways of pattern designs to try and make seamless patterns effortless and after watching some Skillshare classes where they used Illustrator to make patterns I noticed that when they were using the Pattern tool they had elements that were off the canvas it clicked that that was what we needed to do in Affinity.
The key is using a canvas of the size you want your pattern to be but DO NOT use an artboard. Doing this means you can use the clip to canvas function which allows you to see your assets that aren't on the canvas, just like in Illustrator. It is so simple and was there in plain sight the whole time.
I create a symbol just as you would when using the artboard and Live preview (or pattern tile as above) method but instead of adding the extra symbols to the live preview (on another artboard - the old way) I duplicate them around the edges of the canvas (this can be done to create full or half drop patterns in the same way as the old live preview way). I then just add assets or draw as you would to the original symbol, the beauty is they will appear in every symbol and won't be cut off like they are when clipped to artboards. Using the preview button at the top right to 'clip to canvas' shows you only the main pattern tile, any asset that goes over one symbol in to the next will be shown within the tile, click the preview button again and you will see all your symbols and how your pattern will look. Export the canvas as png/jpeg as you would normally for use in POD etc.
I have created myself templates for full drops, half drops, scallops, ogee, brick, diamond patterns (both reflected and unreflected) in this manner. Best of all I never need to duplicate and transform assets.
Affinity pattern.mp4