Mojave Slowed iMac, Disabled WiFi & Time Machine Access

Upgrade to Mojave has caused my iMac to run extremely slowly, taking minutes to even login. WiFi icon now displays “x” and says “WiFi: No hardware installed” when clicked on; the Ethernet connection is not recognized. Entering Time Machine to do a Restore appears to allows access only to dates since the upgrade, and even though previous backups under High Sierra are listed on the right-hand side of the screen, they are not accessible.

I’ve verified that the Time Machine external drive and data are intact by connecting it to another Mac, and I’ve tried using different USB ports on my iMac to eliminate a bad port as the problem. The TM external drive is not appearing in Finder. Using the Disk Utility, however, I can see the drive and the Time Machine volume itself below it, but that volume is not mounted and does not respond to the Mount button.

I’ve reset SMC and NVRAM. I’ve restarted in Safe mode, which was also exceeding slow. Available Memory (9 GB of 16GB) and Drive space (1.4TB of 2TB) are not issues. I’ve checked the Activity Monitor and do not see any processes eating up CPU (95%+ available). FYI, I run Bitdefender antivirus and have a Bitdefender 2 box sitting behind the router for WiFi (in case that makes a difference).

I do not have a Recovery disk to reinstall macOS, either Mojave or High Sierra, although I believe that can be created on another Mac if that’s the best thing to do. (Since my iMac can’t access WiFi, I understand I can’t make that a bootable disk, which would allow me to do a clean install.) I’ve seen other replies here mention posting an EtreCheck log; I don’t have that app on my iMac.

Thanks in advance for your help in solving this dilemma.

iMac (Retina 5K, 27-inch, Late 2015); 3.3 GHz Intel Core i5; 16GB 1867 MHz DDR3; 2TB Drive