This thread may be of interest to you as a general discussion about upgrading across multiple major release versions. From the sounds of it, you've been porting your own special builds, so that could add more complications... hopefully you've proven that your builds work as expected on identical hardware that you have locally.
Personally, I would recommend against attempting the upgrade remotely, unless you have someone on-site who would have the necessary equipment (ethernet connected computer, sometimes a serial USB adapter) and is either directly knowledgable about routers and OpenWrt, or can assist you over the phone or similar by following precise directions should something go wrong. Further, if this is the main gateway for that remote site, it will be important that there is some alternate internet connection (or method) by which someone on-site could download any necessary files and communicate with you.
Keep in mind that the config files from 19.07 are not compatible with 23.05 -- there have been several significant changes that will necessitate configuring from scratch. As such, you may want to consider baking-in all of the required packages and configurations to ensure that the default state is as desired.
Before making any attempts at this, make sure that you have weighed the risks/benefits of a remote upgrade (including the possibility that you may cause the internet connection to go down, if applicable, and need to travel to fix it).