How to set permanently a Gb/s ethernet port to 100 Mb/s for Wireshark monitoring

Want to use Wireshark and a passive tap on a link between Fedora hosts that have Gb/s ethernet ports, so need to slow the link to 100 Mb/s. Can set the 100 Mb/s and see packets on Wireshark after using ethtool to force that speed. But the methods I’ve found on forums and other websites to make the 100 speed persist through a reboot are a bit beyond my skill level as those methods involve writing scrips and getting them to run from systemd, or something like that. Is there a config file that could contain the same information as the arguments and values of the ethtool method?
Thanks in advance.

You will need to add a systemd unit that runs ethtool command.
I am guessing this is what you refer to as too hard?

I doubt it would be any easier, but if you switched from NetworkManager to systemd-networkd, then you could create a config file under /etc/systemd/network containing the following.

[Match]
Type=ether

[Link]
BitsPerSecond=100M
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That should be a .link file according to man systemd.link. These files are handled by udev whether you are using systemd-networkd or not. NetworkManager may, however, undo that setting later; I don’t know if it does.

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