True, but some banks consider the first 30 or 60 days to be effectively new.

Check out Boggleheads and the 3-fund portfolio for what specifically to invest in.

Personally, being in a very similar spot to you, we are not paying early. Instead, we put as much as we are comfortable with into the market and the rest into T-bills and I-bonds.

Also, note that you are comparing your 4% return at 6 years to the total mortgage term (30 years?). You clearly make more money at 4% rates than at 3% costs. The issue is that the 4% isn't guaranteed and the 3% is.

As well, if you have to borrow to pay something, it's hurting someone else who could have gotten me for their money, but they gave it to you instead. So if you want to borrow to pay it off, at least give them the rates they could have earned by putting it into a HYSA or T-bill, etc instead.

Finally, don't forget that insurance and taxes still get paid when the mortgage is done.

Not the best. If you aren't willing to walk though, then you are stuck. If you can prepay the entire loan at any time and no cost, you can refinance later if you find a better rate.

Cap1 Savor one is 3% for groceries, dining, Streaming. Amex is 6% for groceries with $95 annual fee. Amazon prime card is 5% for Amazon and whole foods, etc. Look at the credit card reddit for tons of info.

As for Affirm, plan to pay off the bill at least 30 days before the due date in full and have payments on auto and it's fine. Most people aren't that responsible and they miss a payment or can't pay in full by the end of the 0% period.

Are you saving at least 30% of gross for your own retirement? Are you able to pay other obligations without taking on debt? Do you have all your debt paid off/a plan to pay it off (if low interest)?

Money is there to be used, as long as it is done responsibly. Some people with that income use it for fancy travel or a second home or a boat. If you want to use it to send a willing kid to boarding school and then college (so 8 year commitment), go for it.

Save as much as you can. Look at the wiki for Order of how to do that.

For a home loan, they look at gross income, not net. You'll be fine there. Same for a car loan.

And health insurance! And any taxes if you're not including escrow in that mortgage amount.