Selfhosted

Hi folks, I know many of you are elite system admins running custom built NAS solutions networked together with servers tucked in every spare closet and space in your home, which is awesome. Having said that, I am still newer in my self hosted journey and my existing knowledge is more from running Linux as an daily driver OS since 2005 rather than actually hosting anything. For this reason, even though it's not ideologically pure, I opted for a SynologyNAS for simplicity of management. This was the next step for me after dipping my toes into self hosting after messing around with some VMs and an old laptop.

With the new DSM update, Synology removes several apps and codec support, most notably h.256. I experienced something similar on Linux where I cannot view videos recorded on my action cam. I don't know how many of these photos and videos I have in my file system, but my NAS is local network only and basically contains my photos, videos, ebooks, documents, etc. in separate shares containing a hierarchical folder structure.

My questions:

1. How can I most easily search my NAS for files needing the removed codecs so I can gauge how much this will actually effect me? I want to approach the problem in a simple way that I can understand.
2. With Linux and Synology DSM both dropping codecs, I am considering just taking the storage hit to convert to h.264 or another format. What would you recommend? I havent recoded video in ages so I'm learning from scratch, but I do have a desktop with dual 1080s that should be up to the task.
3. I access my shares via dolphin on KDE. When it comes to thumbnails for a remote filesystem like this are they generated and stored on my PC or will the PC save them to the folder on the NAS where other programs could use them. I just want to make sure I can visually browse the videos and photos on my NAS and have them show up appropriately.

I'm a bit frustrated and kind of favoring just moving things to a different format. I bought a Synology device for an easier experience, and having said that, even if I built a custom solution, didn't Debian remove h.265 as well? I will probably do a TrueNAS or whatever at some point, but I've had way to many family events in the last few years and have to take an easier path right now.

My Linux knowledge is intermediate and my self-hosting knowledge is still fairly basic.