- How much you have to drive. The more you drive, the more you can offset the embodied carbon footprint of the EV, and the more fossil fuels you offset.
- How fast your decades old ICE car deteriorates, and therefore decreases in efficiency and increases its operational pollution emissions.
- The embodied carbon footprint of the vehicle parts you replace through maintenance of an old vehicle.
High range EVs with large batteries cancel their embodied footprint in 18 months with a typical drive cycle, even lower if you are mostly using renewably sourced electricity. Lower range EVs take even less time 
It sounds to me like from your situation (old car, renewable power available) the numbers already suggest you should do it now. From the report linked below, driving an EV in Texas today is like getting 52 mpg with an ICE car. If you're using only renewables it is multiple times more efficient (see equivalents for CA and NY)