Cartoon summarizing the scenario of merger-driven BH growth in FFB galaxies at cosmic dawn. (a) Feedback-free starbursts in thousands of clusters at z​ ∼ ​10, each forming in less than 3 Myr with a typical mass ∼10⁶ M_⊙, in ∼10 generations of ∼10 Myr each over a global FFB period of ∼100 Myr. (b) Rapid core collapse in each cluster during ∼3 Myr, sped up by the presence of massive stars and by cluster rotation and flattening, leading to a very massive central star (VMS) that becomes an intermediate-mass BH (IMBH) of ∼10⁴ M_⊙. (c) Tidal evaporation of the clusters, progressing inside-out in the galactic disk over ∼100 Myr, producing a background stellar disk capable of exerting effective dynamical friction. (d) dynamical-friction-driven inspiral of the BHs that accumulate at the galactic center total BH mass of ∼10^7 ± 1 M_⊙ in ∼0.5 Gyr, namely by z​ ∼ ​7. (e) SMBH growth by BH mergers, subject to GW recoils during the first stages of growth. The SMBH is retained if the galactic disk of BHs is cold or if the potential well is deepened by wet compaction events.