Increasing the number of bins in DFT with limited points and high samplerate
If all you want is a finer bin spacing, then the most straightforward thing to do is to increase $N$ by zero-padding. Note, however, that this will not increase your frequency resolution. Frequency resolution is limited by the total length of time that you observe the waveform. So, if you want to actually be able to resolve frequencies more finely, you need to observe the signal for a longer period of time.

Your first option (essentially interpolating the signal to a higher sample rate) won't do what you want at all; $N$ will be larger, but so will the sample rate of the interpolated signal. If you double the size of the dataset by interpolating by 2, then you'll have to do a DFT of twice the length in order to get the same bin spacing.

Downsampling the signal to a lower sample rate as you suggested in your third option can make sense if, as you said, you only care about frequencies below 300 Hz and your initial sample rate is 44.1 kHz. However, what I said above applies: this won't improve your frequency resolution at all; you'll need to either live with zero-padding to interpolate the DFT spectrum, or observe the signal for a longer period of time instead.