Well, unless you choose to shorten the wiring for your GPS as an example, you have no choice but to zip tie and bundle as I see you've already done. I've done the same, as I suspect many others have as well.
But to attach them to the 1300's wiring I used the
OEM Quartet Harness. That link is to a site catering to the police accessories for the ST1300 which has generally had it available to order, though at the moment they don't. They come and go on various sites, so if you have the time and are patient you should be able to source one. Here's a link to the installation instructions on my Google Drive.
Honda Quartet Harness & Power Plug Installation Instructions_2
It allows you to plug into an unused 9-pin connector below the left fairing pocket and attach up to 4 accessories with combinations of switched and unswitched power feeds. Honda intended these to be used for their accessory heated grips, accessory power outlet (typical cigarette lighter plug socket) and radio.
I sourced a set of Hitachi connectors and terminals from Amazon to connect the wiring from my GPS, heated grips, and USB power outlet in place of the socket.
2 3 4 6 9 Pin Plug Housing Pin Header Crimp Electrical Wire Terminals Connector
Stay within the expected power levels for these accessories and you should be fine regarding loads.
Go beyond this and you'll want to consider a third party wiring block like the
PC-8 Fuse Panel from Eastern Beaver or the one by
Fuzeblock.