I am using an iMacPro and I find it acceptable for LR with ~ 150,000 Raw or tiff files on an external Promise RAID array, and my catalogs and my back up catalog files on my SSD boot drive. I back up my SSD boot drive and my external array to several other hard drives every few days.
I think you may find the USB 3.0 access to be slow enough to be a noticeable link - If your machine offers a Thunderbolt port I think that would be a better choice for connecting external drives. USB 3.0 isn't terrible but for a large number of 20-40 Mb files it will take some time, it will not be near instantaneous, and what you will find you want is - instantaneous.
Lightroom doesn't demand that much power for simple editing of exposure, color balance, cropping, etc - BUT when you start doing local real time selection with adjustment brushes, then the need for power is more apparent - by power I mean processor and video processor power. High speed RAM helps also.
I have not had issues specific to Mojave on my present machine or a 2013 Trash Can Power Mac. I am able to run LR along with PS and several add on plug ins for LR or PS, as well as Safari with 5 or 10 pages in Tab mode. But there are times I close everything but LR too.
I prefer my catalog on the fastest drive access I have, which is my boot SSD, to minimize my waiting for the catalog to load and run, and update as I work.. If your Catalog is large, ( my Catalog is 2.2 Gb, and my Previews file is about 52 Gb ) loading them both can take a few moments even on a fast machine. If the files are on an external drive, they will be much slower to load and run. If they are on your boot drive then you must back up your boot drive often to keep your Catalog safe, and you might prefer your Catalog BU files to be on your external drive storage, or off site on Back Blaze or both.
If you have more specific questions I will be happy to try to post my opinion FWIW.
One other point, I use only ONE catalog for all of my image files, so that with keywords I can find my selected images quickly. Some folks try to use seperate Catalogs, but I don't favor that approach. Nor do I favor scattering my image files across mulitple external drives scattered about, I strongly prefer ALL my image files on one specific Physical drive ( or Array ). That way I always know where they are and where they are supposed to be.