Is the EX141 a good upgrade from an older Wi-Fi 5 router?

Yes, if your old router is a basic AC1200 or older unit. Wi-Fi 6 adds better handling of many devices at once, and the gigabit ports let you use a fast line. It will not match an AX3000 router on peak speed.

Does it work with fiber and cable internet?

Yes. It has a gigabit WAN port that connects to a fiber ONT or cable modem, and gigabit LAN ports for wired devices. Your ISP may need the router set to DHCP or PPPoE.

How many devices can it handle?

It is fine for a small household with phones, a TV and a laptop or two. A home with dozens of active devices, heavy gaming or large file transfers will do better with an AX3000 or tri-band router.

Does it have a USB port?

No. There is no USB port, so you cannot attach a drive or printer to share over the network. Use a network drive or a separate print server if you need that.

Can I use it as an access point or with a mesh system?

It can run in access point mode when connected by Ethernet to another router. It is not part of TP-Link's Deco mesh line, so it will not roam seamlessly with Deco nodes.