Hi
The Tor service might not be able to detect if your private usage is high or not. I would suggest the solution at OS level. Typically congestion balances according a fairness modell which stands against your intention.
what alternative methods might exist to de-prioritize Tor traffic during bursts from personal traffic pipes on Linux or BSD systems?
On BSD you could investigate in pf-altq and define customized queues. But that will impact the bandwidth measurements and the consensus for your relay.
-- Cheers, Felix