The SU/Ostron standing cap should be 2000*MR+1000 since you have to actively earn it, unlike the other syndicates where it's passively earned via sigils. : r/Warframe
Warframe Syndicate Guide: How to Maximize Your Standing Gains
Syndicate medallions can no longer be redeemed past the daily standing cap : r/Warframe
Quills and Ostron standing needs a high-end recursive reward... - General - Warframe Forums