Sorry, not aware of one. It wouldn't be too hard to write some kind of a one-time catchup program if you know the database format the weewx plugin you're using to get your realtime data into weewx. You likely want to be consistent so your grafana dashboards can pick any time range you have data for.
Basically you'd do a select on your archive table and postprocess the output into the format influxdb wants to see for batch uploads of records. Definitely doable with a few hours work I think.
(if you don't get there by Christmas, ping me via email and I could maybe give it a try that week when I'll have time available)
It's not weewx-centric and it's more IoT mainstream, but no, not really in your case. If what you have works, keep using it.
The downside of telegraf/MQTT is that I've found that sometimes it loses the subscription and data doesn't come in. For my raspi IoT sensor things here and my former WeatherFlow gear, I changed my code to write to influxdb directly, which I'm guessing is what Matthew does.
For my Davis VP2 weewx=>influxdb stuff I stuck with the MQTT extension and the telegraf path, mainly because it was stable. Once I got it working locally, I figured "if it ain't broke don't fix it".