  Ultracold potassium is an interesting candidate for quantum technology
applications and fundamental research as it allows controlling intra-atomic
interactions via low-field magnetic Feshbach resonances. However, the
realization of high-flux sources of Bose-Einstein condensates remains
challenging due to the necessity of optical trapping to use magnetic fields as
free parameter. We investigate the production of all-optical $^{39}$K
Bose-Einstein condensates with different scattering lengths using a Feshbach
resonance near $33$ G. By tuning the scattering length in a range between $75\,
a_0$ and $300\, a_0$ we demonstrate a trade off between evaporation speed and
final atom number and decrease our evaporation time by a factor of $5$ while
approximately doubling the evaporation flux. To this end, we are able to
produce fully condensed ensembles with $5.8\times10^4$ atoms within $850$ ms
evaporation time at a scattering length of $232\, a_0$ and $1.6\times10^5$
atoms within $3.9$ s at $158\, a_0$, respectively. We deploy a numerical model
to analyse the flux and atom number scaling with respect to scattering length,
identify current limitations and simulate the optimal performance of our setup.
Based on our findings we describe routes towards high-flux sources of
ultra-cold potassium for inertial sensing.
