5) define the boot order
now I can set the boot order with the command:

sudo raspi-config
chosing Advanced options > Boot Order
Schermata-2022-04-23-alle-11-28-22.png

6) set the root PARTUID of SSD
On SSD I can now set the new PARTUUID for root volume:
sudo nano /boot/cmdline.txt
and replace the SD PARTUUID to the SSD PARTUUID:
root=PARTUUID=9efc3dd8-01 root=PARTUUID=0860e500-02

7) define the mount point on SSD
sudo nano /etc/fstab
fstab.png

Now, rebooting I have this:
astroberry@astroberry:~ $ lsblk
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda           8:0    0 465.8G  0 disk 
├─sda1        8:1    0   256M  0 part /boot
├─sda2        8:2    0 146.5G  0 part /
└─sda3        8:3    0   319G  0 part /media/astroberry/astrofoto
mmcblk0     179:0    0 116.2G  0 disk 
├─mmcblk0p1 179:1    0   256M  0 part /media/astroberry/boot
└─mmcblk0p2 179:2    0 115.9G  0 part /media/astroberry/rootfs
where "sda" is the SSD and "mmcblk0" is the SD.
If I remove the SSD it boots with SD and I have:
astroberry@astroberry:~ $ lsblk 
NAME        MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
mmcblk0     179:0    0 116.2G  0 disk 
├─mmcblk0p1 179:1    0   256M  0 part /boot
└─mmcblk0p2 179:2    0 115.9G  0 part /


The speed is improved:
with microSD (Kingston SDCG3/128GB Canva Go! Plus)
Raspberry Pi Diagnostics - version 0.9
Sat Apr 23 11:51:42 2022

Test : SD Card Speed Test
Run 1
prepare-file;0;0;36187;70
seq-write;0;0;35989;70
rand-4k-write;0;0;8046;2011
rand-4k-read;17151;4287;0;0
Sequential write speed 35989 KB/sec (target 10000) - PASS
Random write speed 2011 IOPS (target 500) - PASS
Random read speed 4287 IOPS (target 1500) - PASS
Test PASS

with SSD (very used Samsung 860 EVO 500GB)
Raspberry Pi Diagnostics - version 0.9
Sat Apr 23 10:32:33 2022

Test : SD Card Speed Test
Run 1
prepare-file;0;0;258015;503
seq-write;0;0;257003;501
rand-4k-write;0;0;52809;13202
rand-4k-read;41217;10304;0;0
Sequential write speed 257003 KB/sec (target 10000) - PASS
Random write speed 13202 IOPS (target 500) - PASS
Random read speed 10304 IOPS (target 1500) - PASS
Test PASS


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