What software is recommended to replace iDVD?

There are no 3rd party apps that can hold a candle to iDVD. If you're running High Sierra you should be able to run iDVD without a problem. iDVD 7.1.2 is fully compatible with High Sierra. I have an external HD with High Sierra on it so I can boot into it and run iDVD when I need to.

If you only use movies to create video DVDs there are quite a few apps in the App Store that can do the job. Many have free or demo versions to try before purchasing. Most will put a watermark on the video.

If you use still photos to create slideshows in iDVD then there's only one app I've found that can use still photos - DVD Maker Lite - DVD Creator.

None of the possible candidates is as intuitive as iDVD is. They all have a learning curve to find out where are the features are hidden.

Regardless of what app you use follow this workflow to help assure the best qualty video DVD:

Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.

To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.

Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.

The best I can do is...

https://burn-osx.sourceforge.io/Pages/English/home.html

Or...

https://www.roxio.com/en/products/toast/titanium/

https://www.roxio.com/en/products/mydvd/?utm_source=bing&utm_medium=cpc&utm_term=osx%20toast&utm_campaign=Roxio_Bing&msclkid=0b79f91a63af1ea5a2aba447fabd2cdb

The disc I burned just from my portable disc drive (with all my movie clips from my iphone) read as data on my TV.

That's because it was a data disc. As said previously one needs a video DVD authoring app to create a video DVD disk that will play on a set top DVD player.

Make sure you didn't move the clips from the Desktop before you burned to disc. Also save as a Disk Image so you can test it before burning as I stated earlier.

Since you've created the project and added the clips have you edited any of the movie clips?

Read Bengt Wärleby's reply in this topic: Multiplexing Error - Apple Community for any possible solution.

What resolution are the clips?

Do you know the size of all these clips? As I recall a regular DVD only holds a 720p movie, Blur\Ray needed for 4K, not sure about DVD/DL.

Good luck Judy, you might be on the right track. :)