I need a simple way to bring lan and wireless devices up and down in a script. Hence ifupdown. I have configured /etc/network/interfaces and NetworkManager.conf as shown below. What am I missing?

uname -a
Linux Bill-Ubuntu 4.15.0-118-generic #119~16.04.1-Ubuntu SMP Tue Sep 8 14:45:47 UTC 2020 i686
i686 i686 GNU/Linux

cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
iface enp8s8 inet static
address 192.168.1.165
netmask 255.255.255.0

cat /etc/NetworkManager/NetworkManager.conf
[ifupdown]
managed=true

ifconfig
enp8s8 Link encap:Ethernet HWaddr 00:16:d4:32:8c:3d
inet addr:192.168.1.165 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::b521:b762:ca4a:d0c6/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:6459113 errors:0 dropped:0 overruns:0 frame:0
TX packets:1234859 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1557602946 (1.5 GB) TX bytes:137259083 (137.2 MB)

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:20152 errors:0 dropped:0 overruns:0 frame:0
TX packets:20152 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1842616 (1.8 MB) TX bytes:1842616 (1.8 MB)

wlan Link encap:Ethernet HWaddr 7c:a7:b0:be:e5:8f
inet addr:192.168.1.175 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::23d4:aee5:eb73:11a9/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:8524 errors:0 dropped:126945 overruns:0 frame:0
TX packets:56 errors:0 dropped:5 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1589094017 (1.5 GB) TX bytes:1612227 (1.6 MB)

sudo ifdown enp8s8
ifdown: interface enp8s8 not configured

Might have to restart network manager. Also try

sudo nmcli con up enp8s8
sudo nmcli con down enp8s8

Thanks for your suggestion of using nmcli to take devices up or down. Initially when I took any device down, it would automatically restart. I solved the wireless part by modifying the connection to NOT automatically connect when available. However I still have a problem with the ethernet. Whenever I take the device down, it comes back up a couple of seconds later. Any ideas how to prevent it from restarting?