  The LIGO-Virgo-Kagra collaboration (LVC) discovered recently GW190521, a
gravitational wave (GW) source associated with the merger between two black
holes (BHs) with mass $66$ M$_\odot$ and $>85$ M$_\odot$. GW190521 represents
the first BH binary (BBH) merger with a primary mass falling in the "upper
mass-gap" and the first leaving behind a $\sim 150$ M$_\odot$ remnant. So far,
the LVC reported the discovery of four further mergers having a total mass
$>100$ M$_\odot$, i.e. in the intermediate-mass black holes (IMBH) mass range.
Here, we discuss results from a series of 80 $N$-body simulations of young
massive clusters (YMCs) that implement relativistic corrections to follow
compact object mergers. We discover the development of a GW190521-like system
as the result of a 3rd-generation merger, and four IMBH-BH mergers with total
mass $~(300-350)$ M$_\odot$. We show that these IMBH-BH mergers are
low-frequency GW sources detectable with LISA and DECIGO out to redshift
$z=0.01-0.1$ and $z>100$, and we discuss how their detection could help
unravelling IMBH natal spins. For the GW190521 test case, we show that the
3rd-generation merger remnant has a spin and effective spin parameter that
matches the $90\%$ credible interval measured for GW190521 better than a
simpler double merger and comparably to a single merger. Due to GW recoil
kicks, we show that retaining the products of these mergers require birth-sites
with escape velocities $\gtrsim 50-100$ km s$^{-1}$, values typically attained
in galactic nuclei and massive clusters with steep density profiles.
