After doing some more digging and applying all sorts of changes to the settings, my problem has shifted: I now assume I have an issue with (ISC) DHCPv6 instead. After the PPPoE connection gets dropped, LAN clients simply won't get IPv6 adresses assigned anymore. IPv6 connectivity on the OPNsense machine itself, however, is fine. So, once I notice IPv6 connections start failing (ie. every 24h), all I need to do is to restart that one service and everything works as expected again (Windows clients might still need a quick ipconfig /renew6). While trying to troubleshoot I've come across several threads in this forum and on github that are explaining this exact issue. However all the mentioned workarounds I have found so far (such as checking Interfaces -> Settings -> Prevent release) don't seem to do anything in my case. I guess, I'll try working around this issue with a script that restarts the isc-dhcpd6 service whenever the prefix changes.
Edit: The issue is solved.
So, I've switched from ISC to KEA to dnsmasq to assign IPv6 addresses via DHCPv6. For whatever weird reason, I don't have the described problem(s) with dnsmasq. As is probably obvious, I lack the necessary understanding of any of this to make this make sense (especially when in theory the configuration was all the same, DHCPv6+managed RA+static leases), but all that matters to me is that it finally works as intended now.
Edit: The issue is solved.
So, I've switched from ISC to KEA to dnsmasq to assign IPv6 addresses via DHCPv6. For whatever weird reason, I don't have the described problem(s) with dnsmasq. As is probably obvious, I lack the necessary understanding of any of this to make this make sense (especially when in theory the configuration was all the same, DHCPv6+managed RA+static leases), but all that matters to me is that it finally works as intended now.