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We present direct N-body simulations, carried out with Nbody6++GPU, of youngand compact low metallicity star clusters with $1.1\times 10^5$ stars, avelocity dispersion of $\sim$ 10 $\mathrm{km\,s^{-1}}$, a half mass radius$R_h=0.6$ pc, and a binary fraction of $10\%$ including updated evolutionmodels for stellar winds and pair-instability supernovae (PISNe). Within thefirst tens of megayears of evolution, each cluster hosts several black hole(BH) merger events which nearly cover the complete mass range of primary andsecondary BH masses for current LIGO/Virgo/Kagra gravitational wave detections.The importance of gravitational recoil is estimated statistically. We presentseveral possible formation paths of massive BHs above the assumed lower PISNemass-gap limit ($45 M_\odot$) into the intermediate-mass BH (IMBH) regime ($>100 M_\odot$) which include collisions of stars and BHs as well as the directcollapse of stellar merger remnants with low mass cores. The stellar evolutionupdates result in the early formation of higher mass stellar BHs than for theprevious model. The resulting higher collision rates with massive stars supportthe rapid formation of massive BHs. For models assuming a high accretionefficiency for star-BH mergers, we present a first-generation formationscenario for GW190521-like events, a merger of two BHs in the PISN mass-gap,which is dominated by star-BH mergers. This IMBH formation path is independentof gravitational recoil and therefore conceivable in dense stellar systems withlow escape velocities. One simulated cluster even forms an IMBH binary(153$M_\odot$,173$M_\odot$) which is expected to merge within a Hubble time.

Black hole mergers in compact star clusters and massive black hole formation beyond the mass-gap