So what here determines if the score goes up instead of down? I’m planning on getting a mortgage soon but wife’s credit score is in high 600s. If we can make it past, 720, we are told we’ll get a better rate. So I’m curious as to how to bump up the score.
Then there's things like age of oldest account, average age of accounts, etc. You may be able to add her to your accounts to improve this factor, if your accounts are older than hers.
Then balances. You get dinged for having a balance on too many accounts. I think you also get dinged for having zero balance everywhere. I think 'ideal' is 3ish cards with balances (you can (and should) pay the whole balance every statement, reporting is usually done just after the statements). Highest ever balance should be less than credit available or you get dinged; ask for credit increases on cards where you ever went over the limit. Total balance shouldn't be more than some % of total credit, I think 35%? You also get dinged for high % current balance on any one card. Again, you can add her on high limit, low usage cards to help her score. She'll get more score points as a joint account holder than an authorizer card holder.
You get dinged for recent (6 month?) credit inquiries, but inquiries are grouped, so if you apply for new credit, try to get it all done in a few days; mortgage inquiries have longer to be grouped.
If you know when your creditors do reporting, you can adjust your payment dates to tweak things. You might make a payment to your credit card before the statement closes even in order to show a lower utilization % and that could move your score a lot depending on details.
Find a way to pay down revolving balances to zero.
If you have cash sitting around, great. Otherwise borrow from a 401(k), friends or family, send non-reporting payments in late et cetera. Running balances rapidly to zero tends to cause a 50 to 100 point bump. After you’ve got approval, rebuild the balance and use that credit to repay the loan. In summary: transfer the debt from reporting to non-reporting sources.
If you’re thinking longer term, find bullshit collateralised reporting loans and take them out. Securities-based loans, HELOCs. Low rate. Manufactured borrowing. But it’s a credit line paid back on time. The algorithm likes those.