SVG is just a type of file that handles "Illustrator" path types like Béziers. Those online font makers accept SVG files to make the character paths.
A "Font" is just an ordered collection of (related) paths (for those that don't get why fonts are being used for 3D).
Since you've created a font, I have some tips for building model sets.
1) Use the highest Em Square allowed (TTF prefers 2048, but allows 8196 - use 8196 — if you have Fontographer, it only allows a max of 8000). Motion renders much lower resolution (w/r/t the shape's edge) with lower em-square settings.
2) use Zero-Width characters!! expecially when you need to have shapes align to each other at a specific point.
3) Set the Ascent and Descent to be equal!
4) Set leading to 0
5) Design your characters, where necessary, so that they "fit" each other in the font making application where you have parts that need to "mesh" in Motion. As an example: a sword. One shape will be the grip, one shape a "guard", and a blade shape. When you design them in the font app, and you type the three characters to preview, they should show the assembled sword shape.
6) DO NOT DESIGN CHARACTERS in the character spaces for A-Za-z!!! All Mac Apps use the font characters when listing the font name in menus. You can use everything else. It helps to use the numbers 1,2,3,... then Shift numbers !,@,#,... then Option numbers ¡,™,£,... then option-shift numbers ⁄,€,‹,... for organization! You can also use any other unicode space you want (but stay with the "reserved" spaces like 0x900 to 0xa00, etc...)
------ this rule is not hard and fast. You may have legitimate reason to use A-Za-z —just be aware that the font menus will be affected.
If you can't create zero-width characters, set the "side-bearings" to 0 so that the character goes "edge to edge" in its character space and you can use center alignment with the font.
Keep in mind, as you design shapes, that the "anchor point" is the "Base Point" (where the starting cursor point meets that baseline). If you're designing a book model, you may want to create the rectangle shape to align to the left edge of the character space — rotating on the Y-axis will facilitate the "opening" effect without having to set an anchor point in Motion.
In general, you will use one character per shape in a model. You can duplicate the text objects and change the shape character to keep alignments consistent.
Motion's handling of 3D text is exceptional. You have access to all kind of things: materials, textures, specular lighting, regular lighting, camera effects, etc... (about 100 times more fun and customizable stuff than exists for USDZ models.) There are a few limitations in 3D Text. For one - no actual spheres (you have to work with extrusions). No revolves. Textures are weirdly and unevenly wrapped around "Sides". There's kind of a work around, but it's not easy. The point being, you cannot wrap an image all the way around a cicle's sides (cylinder). You can break up the circle into quarters and try to match the image mapping that way. There may be a few other oddities you'll run into, but if you're one of those that don't take "can't" for an answer, you'll probably do okay, there is still a ton of things you can do in Motion. And ALL of Motion' animation capabilities apply to all things 3D Text just like anything else in Motion.