  We present simulations of the multi-phase interstellar medium (ISM) at solar
neighbourhood conditions including thermal and non-thermal ISM processes, star
cluster formation, and feedback from massive stars: stellar winds, hydrogen
ionising radiation computed with the novel TreeRay radiative transfer method,
supernovae (SN), and the injection of cosmic rays (CR). N-body dynamics is
computed with a 4th-order Hermite integrator. We systematically investigate the
impact of stellar feedback on the self-gravitating ISM with magnetic fields, CR
advection and diffusion and non-equilibrium chemical evolution. SN-only
feedback results in strongly clustered star formation with very high star
cluster masses, a bi-modal distribution of the ambient SN densities, and low
volume-filling factors (VFF) of warm gas, typically inconsistent with local
conditions. Early radiative feedback prevents an initial starburst, reduces
star cluster masses and outflow rates. Furthermore, star formation rate surface
densities of $\Sigma_{\dot{M}_\star} = 1.4-5.9 \times 10^{-3}$
$\mathrm{M}_\odot\,\mathrm{yr}^{-1}\,\mathrm{kpc}^{-2}$, VFF$_\mathrm{warm} =
60-80$ per cent as well as thermal, kinetic, magnetic, and cosmic ray energy
densities of the model including all feedback mechanisms agree well with
observational constraints. On the short, 100 Myr, timescales investigated here,
CRs only have a moderate impact on star formation and the multi-phase gas
structure and result in cooler outflows, if present. Our models indicate that
at low gas surface densities SN-only feedback only captures some
characteristics of the star-forming ISM and outflows/inflows relevant for
regulating star formation. Instead, star formation is regulated on star cluster
scales by radiation and winds from massive stars in clusters, whose peak masses
agree with solar neighbourhood estimates.
