Hi, jbe. I have a few small personal sites built with online Trellix - the main advantage being that, since it's web based, I can make and publish updates from anywhere. But for a business site - even a small one - the limitations far outweigh that one advantage (and you can get that advantage in many other ways).
You've run into one limitation already. With online Trellix you don't have the access to your files on the server that you need to support a paid log-in, or any other way to limit access (password protect, etc.). You could check with your specific host to see if they provide c-panel or any other kind of file manager that allows you to do this with the online program, but I'd be surprised if they do. If you want to perform any other business-type functions (payment for services, shopping cart, etc.), it will just get more difficult.
Online Trellix is specifically built for personal websites as opposed to business. In fact, the host I have my online Trellix sites with forbids any use for commercial purposes. Your host may not be that strict, but the bottom line is that if you use online Trellix for a business site, you'll be using a program that's not meant to build the kind of site you want, and you'll be fighting that every time you decide to do something new. If you're hoping to get people searching for your site, there are also drawbacks in that area.
I'd suggest you look at the "Website Building Software" part of this forum. Since most of us here use CSB, which has recently been discontinued, finding a simple-to-use program that will do what we need it to is a common interest. I think if you read through the descriptions in that part of the forum, you'll find something that's inexpensive, easy to use, and made with small business sites in mind.