There are a ton of possible failure points and we need more information to know anything for sure.
Assuming your source videos are NTSC and you want NTSC output (as opposed to PAL), there is a “16:9” DVD option under Settings > Video Mode > Non-Broadcast > DVD Widescreen NTSC for building the timeline. I put 16:9 in quotes because the actual resolution is 720x480, which is not 16:9, but then the pixels are marked non-square which makes the final display look 16:9. DVD format does not have a true widescreen 16:9 mode. It stretches pixels horizontally to simulate the look.
Here is the additional information we need:
Are the source videos 60fps? If so, that probably accounts for the choppiness. DVD is either 30fps or 60 fields interlaced. Some of the motion data at 60fps must be eliminated to fit into 30p or 60i, which will prevent the motion from looking as smooth.
DVD format is usually MPEG-2 in a TS container. Since Shotcut exported as MP4, the program that burns the DVD must do a conversion. What program is doing the burn, and what are its settings? A lot of things could go wrong in that conversion process.
You mentioned exporting as DVD SD NTSC, but that won’t preserve the 16:9 look as you noticed. There is an option right below it called “DVD (SD Widescreen NTSC)” that should do what you need.
Choppy can happen if there is a frame rate mismatch between the source videos and the DVD rate (30p or 60i). Blurry is because DVD resolution is only 720x480, so it’s never going to look as sharp as the original.
As noted, DVD can’t handle resolutions that high, and is not true 16:9. So the DVD burning program is having to do a lot of conversion to get the Shotcut export into DVD format. Without knowing those settings, literally anything could be going wrong.
This is likely being added by the DVD burning/conversion software.
This should be plenty of space for the 90 minutes of MPEG-2 that are actually written to the DVD. The size of the original MP4 file is irrelevant since it is not written to the DVD.