With an annual spend of around $40K, you will need around $1M invested to support FIRE. You currently have almost $600K invested, which should double in around a decade (rule of 72). Therefore, you should be able to hit your 6-8 yr goal just by continuing to invest as you have been doing + having a paid-off house by that time.
What is really going to drive your FIRE date is how much you inflate your expenses by buying a bigger/fancier house. If you stayed in your current scenario, your $37,500 would probably still continue to be a reasonable annual spend, because that presumably already accounts for home repairs, insurance, property taxes, and the like. But if you move to a different home, you will have both (1) a larger debt to pay off that isn't accounted for in your $37,500 annual spend, and (2) higher ongoing costs that are associated with a larger/more expensive home. Your $6K in net income you are anticipating from renting out the other half of the duplex won't even come close to covering all of that.*
If you want to get a real handle on the impact of a particular house purchase, look into the expenses listed by
@formerlydivorcedmom. In some areas, property tax alone will swallow up your $6K net from the duplex. Plug that additional information in to your $37,500, figure out how you plan to pay off the larger house,** and adjust accordingly.
Bigger-picture: your house is a consumption item -- you are looking to buy one to upgrade your lifestyle, not to provide a future income stream. Therefore, the way to make the best choice is to stop thinking about how much house you can "afford" and focus instead on what is the least-expensive house that meets your needs.
*This also sounds like very little net income from a fully-rented duplex. May want to do a little more research into that aspect before doubling down on real estate.
**You can have it paid off, which would give you a lower monthly nut once you FIRE, but require you to come up with several hundred grand extra over the next 6-8 yrs; or you can plan to FIRE with a mortgage, which allows your current savings to continue unabated but will significantly increase that planned $37,500 spend.