I bought a Lacie portable drive and linksys Velop 3 node mesh wireless system to replace my old Timecapsule. I've been dong manual BU's to the Lacie but would like to connect it to a Velop node via ethernet. Can I do this?

NO.. absolutely not!!

USB to Ethernet dongles are designed for computers and need DRIVERS. No such drivers are available in a dumb portable drive and even if a bit more brains are available in the velop.. linksys meaning BELKIN never bothered to provide drivers for USB drives.

Yes you can plug in a NAS.. that is a smart box with a full computer brain that is able to provide all the protocols required for Macs to backup just like your Time Capsule in the good old days. You could even resurrect a Time Capsule and plug that into the Velop ethernet. A TC or an AC model Extreme which has the LaCie plugged into its USB port and a few other brand routers support Time Machine to USB.

As far as the speed goes for USB 2 port on either the TC or Extreme, you will find treacle running uphill in winter confirms your belief in how slow a USB2 port on an Airport really is.. even if Apple did give it enough brains (barely) to do the job. Note you will have to start the backups all over again.. because Time Machine works differently to network drives cf local drives. You cannot even see the local backup via the USB network connection.

Overall a NAS is the way to go.. we do recommend the Synology and several of us here use them daily. Synology being the most compatible.. having used a couple of different brands. They are expensive. and no.. you cannot plug the LaCie drive into it and do backups.. you will need a proper NAS drive inside the unit.

Or use a suitable router.. other than TC or airport I have tried Asus and Synology routers which have worked.. with more or less success.. in the end this is a waste as you already bought a mesh set.. so even the cheapest current Synology NAS will beat the daylights out of it speed wise.

Any suggestions on the best AV for Macs?

The best is none, since AV software is not needed on a Mac and often does more harm than good when it is installed.

If you are asking about malware, then use an application for that purpose......like Malwarebytes for example.

Macs don't get viruses so you don't need anti-virus software on a Mac. The Mac operating system has built in protection against other harmful software.

Ignore any kind of anti-virus software for a Mac. It is designed for only one purpose.....to separate you from your money.

Malwarebytes.....which is not an anti-virus application......is free for the basic program, which is all that you probably need unless you are on the Internet all day.