First off, the only dumb question is one that is not asked. So ask; that's what the forum is for. The worst thing that can happen is, people won't understand the question. This sometimes happens when the asker finds it difficult to phrase their question in common technical terms (this is not a criticism, but a description of a rather typical communication problem; it's perfectly normal for a non-technical person to have limited command of a technical vocabulary). Which brings us to your question.
You keep saying "setup", but it is not clear whether you refer to installation of OpenWrt on your router or to administration of OpenWrt on your router once it is installed. Those are, to be sure. related tasks, yet they are separate. Getting the minimal (aka core) OpenWrt running on a particular piece of equipment is one thing; making a particular feature of OpenWrt work on that piece of equipment is another. OpenWrt is extensible modular software; there are numerous packages that are not included in the minimal initial installation, but can be downloaded, enabled, and configured IF HARDWARE PERMITS.
OpenWrt in its minimal implementation can work on a bedazzling variety on hardware. Some of that hardware, however, is kinda puny and may or may not be capable of supporting some advanced functionality that requires a lot of processor time or memory.
Next, you say that your current router is over a decade old. You also say that you're planning to upgrade to a fiber connection. This immediately raises a question: will your current router become the bottleneck once you upgrade? Especially since you seem to be concerned about security. Security is based on encryption; encryption is computationally intensive; many old routers simply can't do it at speed due to insufficient processing power.
So here's the plan of action I would suggest in your case.
First, please clarify where you are in the "setup" process. Are you having trouble installing OpenWrt or is your OpenWrt up and running already and you need help with administration?
Second, please be VERY specific in describing your hardware. There's usually a sticker on the bottom or the back of a router that lists the name of manufacturer and the model number. The same model number sometimes has multiple versions or revisions, also noted on the sticker. These are VERY important, especially in the installation phase. Firmware that runs perfectly well on Something-Something v.2 may brick Something-Something v.3 and vice versa (or not; the devil is in the details). If you have OpenWrt up and running already, you may want to take a look at the Overview page in the management interface and include what OpenWrt reports under the following headings:
Third, please be VERY specific in describing your problem/issue. What do you want your router to do? What does it do instead? What steps have you undertaken to get from here to there and how did the router respond (error messages, etc.)?
Fourth, be mentally prepared for the possibility that what you want is not achievable with the hardware on hand. (See, for example, the discussion of bottlenecking above.)
And finally: if you want a complete recipe, you need to know what you're cooking. A seemingly simple matter: you keep talking about "security", but does your definition of "security" include, for example, parental controls? If you can't state your specific wants, we can't help you figure out if they are even achievable... Again, this is not a criticism, but an invitation to work together and overcome the inevitable difficulties.
With all this in mind, welcome to the forum!