Writing an editor from scratch is a lot of work. That's one reason why I would use an existing editor whenever possible. For example, TrenchBroom (Quake editor) + func_godot seems to be getting more popular among Godot users, and Tiled looks pretty good for 2D games. For game data management I've seen CastleDB (never used it though), which now seems to be integrated into Hide, a full-blown 3D editor.
Either way, once you get the tooling up and running, the next big step is actually designing a game and creating all the content. :)
One thing I particularly like about Krita is its wrap around mode. Very useful when making seamless textures! Another thing is that its file format contains a merged image, which can easily be extracted (.kra and .ora files are zip archives). I've used that to add native .kra/.ora support to a game texture conversion tool. .psd support took a lot more work in comparison.
During the pandemic I got back to an old hobby, creating Half-Life levels. I found that certain things involved a lot of repetitive work, so I started working on some automation tools.
For textures and sprites, I made WadMaker and SpriteMaker, which can convert a directory full of images (including Photoshop and Krita files) to the specific formats that HL uses: https://github.com/pwitvoet/wadmaker/
For creating levels, I made an automation tool named MESS (Macro Entity Scripting System) that can do things like covering terrain with props, simplifying level scripting and automatically applying workarounds for known bugs: https://pwitvoet.github.io/mess/
It's been very educational (and fun), learning about color, geometry and making programming languages.
Either way, once you get the tooling up and running, the next big step is actually designing a game and creating all the content. :)