Filemaker 19 pro ... replacement for Directory Opus but for Linux?

So... this is a long shot but I'd recommend to reconsider how you work. Switching to Linux is already amazing while also being demanding. Still, there are genuine alternatives to pretty everything on any OS, not just Windows.

What is more challenging IMHO but also more interesting... is reconsidering how you work, not just the tools you enjoyed so far. So yes, as others pointed in the thread there are custom file managers (beyond the default or popular ones) but, and please hear me out, there is also the command line. I know... I know it is VERY different but that's a good thing! If you already looked and used an alternative file manager it means you are a power user. The command line (or CLI for short) is precisely a way to have MORE freedom to manager files. There are countless tools that one can combine to modify files. It will take a while to learn but it's definitely worth it. A good starting point could be https://wizardzines.com/zines/bite-size-bash/

For the other software... well if it's from work, even though I'd also suggest to look at alternatives, e.g. learning Python/Tkinter or even low code FLOSS alternatives or Web based one... you might not have that freedom. Consequently I'm going to make an even more outlandish suggestion : if your work does not trust you to pick your own tool... maybe reconsider your workplace? I know, bit crazy but long term, might still be worth it.

Apologies for the life changing suggestions!

Congratulations, but you'll be hard pushed to get windows apps (especially enterprise ones) working. Will probably need to virtualise windows.

Congrats on switching!

Debian, but with latest kernels to improve gaming experience.

Genuine question as I'm a basic gamer but not a super performance oriented one (namely, I click "Play" and I enjoy with basically default settings) what PikaOS kernel version is available now (6.15.0 AFAICT from their wiki) versus current Debian stable (6.1.140-1 (2025-05-22) AFAICT from uname) feature is differentiating for gaming?

I understand, beyond security implication, that having a newer kernel should be "better" in general terms but if I take e.g 6.15 vs 6.1 what actual difference does it makes? Is it like a 1% FPS increase? It is a feature e.g. FreeSync/FSAA/etc that the driver itself require?

PS: I admit it's an in depth question because I have frequent "arguments" about people criticizing the "slow" Debian stable so this is kind of an excuse to understand what I am actually missing.

Although I'm not familiar with FileMaker 19, I was also less than impressed by Winehq but it was instrumental in me getting SketchUp Pro 2023 running (with only a few glaring issues lol).

Which features of Opus do you specifically use that aren't part of regular file managers? KDE's file manager doesn't do what you need? Other ones to try are Konqueror (probably the closest to Opus, it should be part of Debian's repo: sudo apt install konqueror), PCManFM (also part of debian's repo), Krusader, and Double Commander (found online, third party).

As for Windows apps, do NOT try to run them via Wine. Even if they will install (unlikely most of the time), they will be crashy. Games work because they don't use much of the Windows API, but apps do, and most of the API is not implemented under WINE. So your best bet is to run a Windows VM. You can setup a "spice" to share files between your VM and your Linux system, and then run these apps under a free version of Windows (you don't have to pay for a license these days).

I'm not familiar with Directory Opus but try Krusader (sort of a spiritual successor to Norton Commander)