This question may have been answered but still here it is!
I am planning to use Cloudflare tunnels or Pagekite to reverse proxy my next cloud instance and use it outside the network.
Both of these services will install their client on my server and provide their own SSL.
How safe is my data in transit and can these reverse proxy servers (not clients) have access to any data?
This is probably the wrong place to ask there questions… I would look for support place of the product you plan to introduce.
In general - if the traffic between Nextcloud server and Reverse Proxy is not encrypted then RP has full access to all data in transit, depending on the rights of the local client they may even have access to data at rest (data on servers local storage).