One big-picture issue I would suggest you keep in mind is avoiding the trap of trying to buy your way into frugality. E.g., when the first action item is "OMG, I need to be more frugal, so I need to buy a different car!" In your case, that may actually be the appropriate first step; we don't know, because we need to wait for the full case study. But the first thought has to be how you can change your lifestyle overall -- rather than replacing one car with a cheaper one, is there an option to get rid of the car altogether? How's the house/mortgage -- does that contribute to a sustainable, low-cost lifestyle where you can walk a lot of places, or do you need to get in the car to drive everywhere? Etc. You need to get out of the box where spending is the first solution -- again, it might be the right solution, but it isn't where you start.
Second, take your time and do a lot of research. E.g., if your long-term plan is to buy and rent houses, do a ton of research on RE investing so you can spot a good deal and understand the kind of capital you need. There are many people here who FIREd by buying RE at low prices after the crash and finding tenants who are providing a significant profit. But those circumstances did not exist in all markets, and they no longer exist at all in some areas. There is a lot of work between the initial conception of the idea and the execution.
Along those lines, before you jump into a new money-making enterprise, make sure you have your downside covered -- primarily, life and disability insurance, along with wills and a guardian for the little one and the one(s) to come. And you need a good emergency fund -- especially if you plan to go down to one income, and especially if you are going to then invest in something that requires large-and-unknown capital outlays (like, say, RE in need of renovation).
On your specific questions: the CC Q depends on how long the 0% remains in effect and whether you will owe back interest when that rate expires. If you will owe interest back to the date you first bought the whatever, then no matter what else you do, you must have those paid off before that deadline. If there is no back interest but you will just start paying interest on whatever the balance is at that time, then how important that deadline is depends on the new CC rate, your other debt, and other priorities.
Mortgage: Look, this is your life. Financially, it is clearly better to invest the money than pay off such a low-rate debt (those of us who are older and remember the days of 16% mortgage rates are dancing in the aisles at our 3-ish-% rates and have zero desire ever to pay it off). So if you choose to pay it off, do so in the full knowledge that you are choosing current lifestyle -- having a SAHM -- over future profits, and that this choice will delay your FIRE date by X amount. E.g.:
Option 1: pay off mortgage within next 5 years, DW becomes SAHM. Cost of the choice: (growth received from investments through FIRE date) - (avoided interest at 3.75%); this cost delays FIRE by X amount. Benefit of the choice: guarantee that DW can become SAHM.
Option 2: invest prepayment amounts in VTSAX in a tax-sheltered account and let it ride until FIRE. Cost of the choice: need to figure out some other way for DW to SAHM, or both continue to work, or you SAHD instead. Benefit: significantly higher long-term investment growth (growth received from tax-sheltered investments through FIRE date - 3.75% interest paid over remaining term of mortgage) = FIRE much earlier.
Option 3: invest prepayment amounts in VTSAX in a regular brokerage account for 5 years. At 5-year point, remove amount sufficient to pay off mortgage, DW becomes SAHM. Cost of the choice: if market goes down, DW may not be able to SAHM as soon as you'd like, or you may need to come up with extra money to pay off the mortgage; can't use tax-preferred retirement accounts if need to use money within 5 years. Benefit: (growth received from remaining non-tax-sheltered investments through FIRE date) - (extra interest paid at 3.75% for 5 years).
Etc. -- there are a ton of other options, like downsizing to a smaller home that you could afford on your salary alone. I am not implying that any choice is good or bad, right or wrong; what matters is that you do the analysis so it is a knowing choice that you are both making with a critical eye (avoiding confirmation bias and rationalization as much as possible), and with your values and long-term plans in mind.
