I totally agree that the last SRS interval of 6 months is ridiculously short. I have items on Anki that are due in 4 years.
One easy way to handle mastered items in Bunpro could just be to let the user chose a fixed interval in the settings (let’s say 300 days) after which all burned items will be shown again. Items will still be SRS 12 and don’t accumulate any XP, but they will simply be shown again after the user defined interval (which can be toggled on/off). Items that are marked as “I know this” will be excluded though.