I will leave most of this to those who have a lot of experience/expertise. Only a minority of those on forum do DIY, and it > population it is far,far, far smaller - so not for everyone and does take a small investemnt (>$150 for hardware). The Phonak Target software is free and is the same as what your Costco fitter uses or a private audi would use.
If one undertakes DIY, I understand that a typical approach wold be: 1) Connect hearing aids via that <$150 hardware interface, and then in the Target software save the settings that had been programmed by Costco. This then allows you to change virtually all settings; try it out in real life; and always have the option of reverting back to what the “expert” or at least more experienced fitter had set. 2). Both fitter and DIY is guided by both the audiogram (properly done only in sound proof booth and thus not a DIY item); AND by the comments of the user. It is also guided by what Costco did at your fitting appointment as one step REM (Real Ear Measurement) which put very small microphones in eah ear canal to determine whether in actuality the amount of amplification the programming had chosen as the best fit for your hearing loss (audiogram) was actually being produced by your pair of hearing aids in your actual ear. This verification step is an important aspect of best practices for fitting. Often it does NOT result in any changes as everything is as hoped for; at other times the fitter might need to tweak the amplification by say 1 or 2DB for certain of the frequency bands. REM is not something you can DIY, but often is not needed again unless the type of domes is changed, or your hearing has changed overtime and the needed amplification has changed.
3) I think of DIY as a supplement to what a fitter might do; use it as an option as much or as little or not get into at all. So an example: The default/zero/0 volume that fitter set with my input was raised by 2DB from what the Target software suggested. With my experience (and I’ve worn HAs for nearly 6 years) the default volume would be better 2DB lower than where it is now. Rather than wait for my next followup appt. I could make this simple change myself. As it is and for other reasons as well I am often using the manual volume control to set the volume across all programs 2DB lower - not a big deal or much time to do, but would be nice to “fix” this myself rather than waiting. More important would be for example to make a copy of just the Restaurant (Speech in Noise) program, and then make adjustments to its functioning (noise suppression, etc.) and then save that program. With two options now in my HAs I could then do A/B comparison in a real life setting and determine which was better, or whether something “in between” worked better. So some of DIY is willingness to experiment with different settings (tweaking but not often radical changes from what fitter did); observing/taking notes; making decision. It is an iterative approach that for some leads to minor/somewhat to significant hearing improvement and is worth the effort. Most users as well as their fitter are not willing to invest the time in many appointments over a period of time to make such fine tuning experiments. My own hearing loss is in the Severe category and an optimal fitting is more challenging to achieve and there are more factors and tradeoffs in uncovering what is optimal. As an engineer you might like DIY, and it might not interest you at all.
Posting your audiogram to your Avatar/profile would allow members to have a clearer picture of your hearing loss and thus make any comments more helpful. But remember the audiogram is only showing how your ears respond to frequency/amplitutde input; listening takes place in the brain/mind and how any individual interprets sound/speech depends on how that mental processing takes place. An optimal fitting for two people with identical audiogram and using the same HAs might not have the same HAs settings (each person’s ear canal is a different acoustic environment, and of also great importance is the mental processing that takes place for any signal to “extract” meaningful information from it).
Enjoy your new hearing aids.
Edit to add this: There is no one right way to relate to hearing aids. Some leave virtually all decision making upto the professional; others like the ability to experiment and have independent capacity to adjust; and all variations in between. Everyone is welcome in the forum.