Comparison of timescales for effective migration driven by PR and stellar wind drags (solid black lines), evaluated at an age of 4 Gyr (2-3 Gyr for F-type), with the collisional timescale corresponding to the optical depths τ_BG = 3 and 27 zodis referenced from the HOSTS survey (dashed purple lines; see Sects. 3.2.2 and 4.3). The comparison is made near the position of the planet ∼a_p (HZ_mid). Collisional timescales for a range of face-on optical depths, τ = 1-1000 zodis, are also shown as a purple band (top to bottom). Results are shown for each stellar spectral type as a function of grain size over 0.1-1000 μm, restricted to s ≥ s_BO,eff and adapted from Fig. 3 of Najita & Kenyon (2023).