Mortgage scores with a collection really can’t get much higher than 740 as a general rule. I think one of our forum members with a file far more stacked than mine or yours OP got to around 755 with a tax lien showing which scores similarly to a collection.
30D late only is a bit of a weird one (worst of the top 8 scorecards and my 60D late TU FICO 4 being arguably the best of the bottom 4 possibilities was substantially higher in score at 770+) but I was 730+ with a one year old 30D late at that point on EQ FICO 5 and I am pretty sure EX FICO 2 was higher and I know TU FICO 4 was when it was only a 30D on that file.
The big thing is stop opening accounts, right now and clean up your revolving utilization... those combined with even small numbers of inquiries can be 30 points easy on that scorecard with similar account age to yours. Saw one drop all the way down to 702 EQ 5 in the record after I opened up a pair of revolvers and probably had some accounts with balances issues there too, but EQ decided to whack that tradeline for I still don’t know why and I am back to seriously pretty on that file now, less than 20 points from real world max even with several small blemishes on the file.