Tiktok limits videos to a maximum of 60 seconds. This limit has been increased to 3 minutes but is not widely available as of yet. You can choose 60 seconds of your video from your linear editor and export that segment. Sort of like a highlight. You can upload that directly as TikTok will automatically display your video in letterbox format. Black bars top and bottom. But if you want it zoomed in like a portrait, drag and drop that video over the script and ffmpeg will do the rest. This means you do not have to create a new project in Premiere/Resolve/etc. or open another application to go through another UI just to get the aspect ratio and dimensions you want. Reduces your total input.
In the end, it does not matter if you use 1080p or 4k video. It will be on the platform conforming to TikTok video requirements (re-encoded). If you have no intention of cropping and want to rely on automatic letterbox display, I highly recommend you export your videos using 30 fps. As TikToks maximum bitrate d