Yes, the “slow fill” symptom is probably an EVAP problem. Unfortunately, it’s fairly complex and can get complicated to troubleshoot. The ‘canister vent valve’ may be clogged or defective, which shouldn’t be too expensive to replace.

I’m going to take a guess that the gas gauge problem is voltage-related. The voltage regulator on the alternator may be shot; I’m thinking the voltage is going up with the RPMs of the engine and the gauge reads higher with the increased voltage. I’m not sure if you would notice other things (lights, blower motor) changing as well, though. There also may be a voltage regulator for the dash gauges, but I’m not sure why it would fluctuate with engine speed in that case.

You’ll need a real mechanic, not some kid at a chain shop, to diagnose the problems (I doubt they are related, but it’s not impossible). As far as car repairs go, these might not be too pricey-hundreds, not thousands, of dollars.

Fill the tank up, and set the tripmeter if you have one, or make a note of the odometer reading. drive for, say, 100 miles, fill the tank up, and note how many miles and how much fuel it took. You can figure out how far a tank will last you, and fill up when you get below 1/4 tank or whatever. Reset the tripmeter each time you fill up. If you note the gauge reading before you fill, you should be able to see if it’s consistent, and what it actually indicates.

I had a ‘92 Sentra that had problems starting when very cold (0°F). Turned out it was the coolant temperature sensor (not the gauge sender). It failed so ECM couldn’t compensate for cold weather.

If if runs OK, I’d sell it as-is to somebody that thinks they can fix it on the cheap or run it as-is. You’d feel pretty bad if you replaced the cat and it got stolen before you sold the vehicle.

I had a '92 Sentra I bought from a friend; had 175K on it. He told me he hadn’t changed the oil since he bought it, think it was about 1 year? And he drove lots of city miles, hard, every day.

Was still running perfect when I scrapped it because it rotted out.

The car I have now, 2004 Sonata, 117K, I went 12 months without changing the oil. I don’t drive that much, so was less than 5K. I did drive it 20-40 miles on the highway every month or so. I’m guessing the 2.4 will still be running when it goes to the scrapyard as well.

I doubt you did any damage at all to the Mazda.

I had the same problem with my 2002 Sonata. What seems to happen is the membrane in the charcoal canister that stores the fuel vapor breaks, and the charcoal pellets migrate into the system and clog is. It could be your canister vent valve is clogged or stuck closed. It should be fine to drive, but it won’t pass inspection in some areas, and the “slow fill” is a constant irritation. Here’s some more information on the system:

https://repairpal.com/obd-ii-code-p0442