A catalogue tool does 60% and charges for 100%. A tool built for you promises 100% and, if the scope is not written, charges twice. The decision is not "modern or old". It is which part of your month does not fit a product that already exists.
What off-the-shelf solves
A process many people do the same way. Invoicing the way others invoice. A CRM that is not your edge. If the flow is common and the gap is small, buy. The workaround for that 10% is cheaper than a project.
What it does not solve
The 40% that is yours: the odd rule, the close on the 28th, the number that only exists in your team's head. There the catalogue becomes two systems: the one you pay for and the one you actually use (another sheet, another email, another person).
If that 40% is the month, it is not a module that "will come later". It is the product.
The useful question
Is the process your way of working, or a chore anyone does? Chore: catalogue. Way of working: custom.
Second question: can you write the scope on one page? Inputs, outputs, who uses it, what must not fail. If you cannot, you are not choosing software. You are choosing an open-ended quote.
A fixed price is not "cheap"
Fixed means the scope is written and the number does not move if the scope does not move. There is no figure in this article. If the scope grows, it is another quote. If you will not accept that, catalogue and custom cost the same: you will pay for the change either way.
Where this ends
This page decides. It does not sell the commission. If you already know the process is yours and the scope fits on a page, the next step is custom software. If not, stay with the catalogue and write the 40% down. That page is what will tell you when to switch.