If it were me, I'd have Prestige from non-planet sources decay each half-year by 50%, or two points, whichever is higher. That way, high Prestige from donations can only be maintained as long as you keep donating, and tapers off quickly when you stop. It also effectively puts an upper bound on Prestige gains from other sources.
Hang on, think imma graph this one out.
Kay, did some fake math in Open Calc, and using this formula it looks like you top out around 15 Prestige (just from donating at current rates, no Planets included in figure) assuming you give your entire Supply and Intelligence to the Empire every single quarter, at current rates. You hit 11 after about a full year of donating (in thethird quarter, before that cuts in half and you have to work your way back up. Average is 7.5) In the second year you hit a high of 14, but your average is just 11.72. Third year you pass 14.5 but your average only goes up to 12.77, and if you stick with it and don't get ganked from going all out on donation Prestige you get reeeally close to 15 in years 4 and 5 (averaging 13.04 and 13.10 respectively) before you finally see the big 15 at Q4 of year 6.
If we alter the formula to cut by 3/4 every half year (i.e only 25% of extra-planet Prestige carries over) then the high mark in the first year is a little over 9 (6.56 average,) second year is very nearly high of 10 (average up to 8.03) and year three and onward is a steady cycle of seeing your Prestige get as high as 10 before dropping back to start the next year, while looking at a consistent average of 8.12 Prestige.
Hm. This is tricky. An ideal solution would be to have each bit of Prestige from donations evaporate exactly a year after it was granted, but then you'd have to track
when all the extra-planetary Prestige was acquired to erase it properly. I suppose this wouldn't be terrible if you're already working off of a calendar/timeline (gave Toron 2 Prestige Q1 1000, make note on Q1 1001 to deduct 2 Prestige
which is also when the facehuggers will attack) but yeaaaah.