Welcome. I hope you can find some good information and support here. There are a lot of smart and caring people here.
Your question about self-assessment is an important one. I don't think we have a solid recommendation for this. If we were to rely on subjective responses, they are confounded by the potentially poor memory we are trying to measure in the first place, and other factors like day to day anxiety and anxiety specific to cognitive performance itself. For objective assessment, options span for online tests of highly varying quality to in person testing with qualified psychometricians or neuropsychologists. The online tests are very alluring because of the ease of access and typically lack of cost. Unfortunately, most of them are not validated with solid research even if they seem on the surface to look like they should be reliable measures. Seeking a trained person to test you is, on whole, the most reliable, but there is the cost, access challenges, overall hassle factor and difficulty in testing as frequent as many would like.
As a first step, I would suggest you simply write down your observations on a regular basis over time. It won't be a comprehensive list of memory or other cognitive failures, but it should give you some insight as to how things might be changing over time. There are basic phone apps with pop-up reminder that are helpful. It does not need to be anything fancy or specific to your situation. The other benefit is that when you discuss your situation with health care providers, you can call up a record over time to help you more objectively recall how you are doing. Of course, tracking such changes as you start, stop or change interventions (meds, supplements, lifestyle changes, etc.) should be noted so that you can see more clearly possible cognitive effects. Just be sure not to jump to conclusions about changes based on only a few data points. Try to be patient even if your are venturing into a temporary "freaking out" stage.
If you have the resources and are concerned you may already be experiencing cognitive problems or are simply at high risk, a visit to a neuropsychologist or dementia clinic would be a great way to determine your current status and set a baseline to more precisely detect changes in the future.
I am not familiar with prograulin so can't comment on that.
Finally, frontotemporal lobe dementia is not often specifically addressed here, but many of the lifestyle interventions focusing on sleep, exercise, a whole foods diet that keeps you insulin sensitive and reasonably lean, stress management, positive social connections, keeping your brain active, etc. can be powerful ways of fostering brain health.