I would recommend learning Blender for many reasons, but for this use case, it seems the popular choice in what few posts there are for material service. It’s quite new to Roblox. There are plenty of resources out there to learn texturing on Blender
Blender is great for modeling, but it has great tooling for creating textures like you would want for this.
Check out Upload Custom Materials in the New Material Service Beta, it explains how to upload MaterialVariants. The Color, Metalness, Normal, and Roughness maps you would create and export in Blender.
Blender doesn’t lag in things with low resolution.
The problem can be downloading.
Use the app PortableApps and download Blender in your Google Drive, that way the Blender will download fast (not on pc, but on Google Drive) and with less lag. (Btw, you can use Blender of PortableApps inside Google Drive)
Now that’s a debate of simple internet safety, but no, Blender is not a virus. Source it from the official website or Steam, both branches are reputable. Steam auto-updates which is nice.
If you choose to use assets from third-parties, sure, the wild west of the internet could certainly house viruses for you to find.