Actually may previous post (long boot problem >5 minutes) was about 2021-003 (not 2020-003).
After 2 additional tries (second try after "clean install" of Mojave), I finally was able to make it work but I dont understand why it failed twice. Actually the two first tries I installed Mojave Security Update 2021-003 + Additional Update and Safari 14 together. Maybee they should be installed separately. If I have some time I'll try the additional update & safari updates later.
Config : MacPro 5.1 ROM : 144.0.0.0.0 / Radeon RX 580 8 Go, Samsung 1TO SSD(s), 32GB RAM, 2 x 3,46 GHz 6-Core Intel Xeon.
If it can help. Other more recent macs (Air, etc). have been updated without a glitch...