Honestly, if you can afford it, using Photoshop is better for making thumbnails for your YouTube videos in my opinion because you have a better degree of control when it comes to design, theme, information, font, text, and colors. Adobe Spark looks interesting; I'll admit I've never used it, but that's the software that YouTube Thumbnail Creator is based on, according to the information provided on the site here:
https://spark.adobe.com/make/youtube-thumbnail-maker/
Through Photoshop, you can add shading, gradients, multiple colors, etc. to your text, best of all, you can adjust the resolution of your thumbnails; highly important when considering high-resolution images. As for text transition, that's a frame-by-frame animation that would basically be a one-way .gif file, and I don't think YouTube accepts those for video cover thumbnails. Unless you have an example of one and can link it below, I don't see it possible to make .gifs possible for thumbnails.