First a tiny feedback to /etc/hostapd/hostapd.conf. There is the ssid line twice. 
Regarding the hostapd and dhcpd issues I tested a more radically way successfully. Actually I am not very happy to see the way how the hotspot.service is implemented. I would suggest to remove this service and do it just with hostapd.service and dhcpd.service / dnsmasq.service.
I changed the /lib/systemd/system/dhcpd.service to:
[Unit]
BindsTo=hostapd.service
After=hostapd.service
Requires=hostapd.service
[Service]
Type=forking
ExecStart=/usr/sbin/dhcpd
KillMode=mixed
Restart=no
StandardOutput=syslog
StandardError=syslog
SyslogIdentifier=dhcpd
I also changed the /lib/systemd/system/dnsmasq.service to :
[Unit]
BindsTo=hostapd.service
After=hostapd.service
Requires=hostapd.service
[Service]
Type=forking
ExecStart=/usr/sbin/dnsmasq --address=/#/192.168.211.1 -2 --interface=wlan0
KillMode=mixed
Restart=no
StandardOutput=syslog
StandardError=syslog
SyslogIdentifier=dnsmasq
Further it has to add a empty /var/lib/dhcp/dhcpd.leases file.
The /lib/systemd/system/hostapd.service I didn’t change at all.
With
systemctl disable hostapd
the the hostapd (providing an access point) should not start at boot time as well as the dhcpd service and the dnsmasq service.
I know there is an extra hostapd-edimax solution checked in the hotspot.sh. Could anybody explain it this still necessary on Buster?
The 8192cu is even blacklisted by default: /etc/modprobe.d/blacklist-8192cu.conf
Another point, please could you change in the /bin/firstart.sh:
mkdir /var/log/samba
to
/var/log/samba/cores
Otherwise there is always an error in the journalctl:
Apr 29 16:58:40 volumio nmbd[595]: mkdir failed on directory /var/log/samba/cores: No such file or directory
Apr 29 16:58:40 volumio nmbd[595]: Failed to create /var/log/samba/cores for user 0 with mode 0700
Apr 29 16:58:40 volumio nmbd[595]: Unable to setup corepath for nmbd: No such file or directory
Apr 29 16:58:40 volumio nmbd[595]: [2020/04/29 16:58:40.802926, 0] ../lib/util/debug.c:1063(reopen_logs_internal)
Apr 29 16:58:40 volumio nmbd[595]: Unable to open new log file '/var/log/samba/log.nmbd': No such file or directory
Apr 29 16:58:40 volumio nmbd[595]: [2020/04/29 16:58:40.802926, 0] ../lib/util/debug.c:1063(reopen_logs_internal)
Apr 29 16:58:40 volumio nmbd[595]: Unable to open new log file '/var/log/samba/log.nmbd': No such file or directory
Apr 29 16:58:40 volumio nmbd[595]: [2020/04/29 16:58:40.807944, 0] ../lib/util/debug.c:1063(reopen_logs_internal)
Apr 29 16:58:40 volumio nmbd[595]: [2020/04/29 16:58:40.807944, 0] ../lib/util/debug.c:1063(reopen_logs_internal)
Apr 29 16:58:40 volumio nmbd[595]: Unable to open new log file '/var/log/samba/log.nmbd': No such file or directory
Apr 29 16:58:40 volumio nmbd[595]: Unable to open new log file '/var/log/samba/log.nmbd': No such file or directory
Apr 29 16:58:40 volumio nmbd[595]: [2020/04/29 16:58:40.812337, 0] ../source3/nmbd/asyncdns.c:158(start_async_dns)
Apr 29 16:58:40 volumio nmbd[595]: started asyncdns process 660
Apr 29 16:58:40 volumio nmbd[595]: [2020/04/29 16:58:40.815455, 0] ../lib/util/become_daemon.c:149(daemon_status)
Apr 29 16:58:40 volumio nmbd[595]: daemon_status: STATUS=daemon 'nmbd' : No local IPv4 non-loopback interfaces available, waiting for interface ...
Apr 29 16:58:40 volumio nmbd[595]: [2020/04/29 16:58:40.816990, 0] ../source3/nmbd/nmbd_subnetdb.c:254(create_subnets)
Apr 29 16:58:40 volumio nmbd[595]: NOTE: NetBIOS name resolution is not supported for Internet Protocol Version 6 (IPv6).
However, I don’t know what might be the impact of this change because /var/log is mounted as tmpfs.
Finally, in terms of your question:
After the changes regarding hostapd and dhcpd services I get only one error: xbindkeysrc.service
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Succeeded.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Service RestartSec=100ms expired, scheduling restart.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Scheduled restart job, restart counter is at 1.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Succeeded.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Service RestartSec=100ms expired, scheduling restart.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Scheduled restart job, restart counter is at 2.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Succeeded.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Service RestartSec=100ms expired, scheduling restart.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Scheduled restart job, restart counter is at 3.
Apr 29 16:58:40 volumio systemd[1]: xbindkeysrc.service: Succeeded.
Apr 29 16:58:41 volumio systemd[1]: xbindkeysrc.service: Service RestartSec=100ms expired, scheduling restart.
Apr 29 16:58:41 volumio systemd[1]: xbindkeysrc.service: Scheduled restart job, restart counter is at 4.
Apr 29 16:58:41 volumio systemd[1]: xbindkeysrc.service: Succeeded.
Apr 29 16:58:41 volumio systemd[1]: xbindkeysrc.service: Service RestartSec=100ms expired, scheduling restart.
Apr 29 16:58:41 volumio systemd[1]: xbindkeysrc.service: Scheduled restart job, restart counter is at 5.
Apr 29 16:58:41 volumio systemd[1]: xbindkeysrc.service: Start request repeated too quickly.
Apr 29 16:58:41 volumio systemd[1]: xbindkeysrc.service: Failed with result 'start-limit-hit'.
But maybe his is not a new issue: https://github.com/volumio/Build/issues/386
