Can I create a slideshow of images to dvd from the Burn to disk command in Finder?

1 - Removing the images from Photos after making slideshow/movie is to not clutter Photos app with such a large batch.

I have them saved in another folder on my MBP.

2 - All my music I obtained through Apple's iTunes.

Since I am now going to use Apple's applications, I can play the new slideshow/movie (with/without music tracks from iTunes) by way of AirPlay and AppleTV to my television set -- right?

3 - If later I want to make a DVD, I would need to save as disk image "using a video DVD authoring application" (e.g. a Toast app.), I presume.

1 - I take it that you do not use a Photos library to manage your photos, right?

2 - since I don't have AppleTV I don't know if the iTunes music will play or not under those circumstances.

3 - you must open the exported slideshow with the video DVD authoring application and create a video DVD project with it. Then burn that project to disc or save as disk image to test before burning to disc

You need a video DVD authoring application to burn a slideshow to a DVD disc to be shown on a set top DVD player. Go to the App Store and search for "video DVD". You'll find many candidates. Most of them have demo versions you can try before purchasing. They add a watermark to the finished product which is removed in the purchased version.

Then export the Photos slideshow at 720p to the Desktop. NOTE: any music purchased from the Music store will not export due to its DRM protection. You can only use unprotected music either from ripped CDs or non-DRM tracks from the Music Store.

First 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 the disk image to disk with Disk Utility 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.

Two answers to your query:

(1) I have seen slideshows with thousands of images done in Photos. It should work. In fact in Photos you can save the slideshow as a "project," which will play like a movie that you can save (you can then copy the movie file to the DVD).

(2) If you don't want to use Photos, you can use Preview. But you have to first select all the files and open them (all) in Preview. Then under the View menu you can select Slideshow (near the bottom of the menu options).

I believe that the .mov file for (1) can be played on different types of PCs. For (2) you must be on a Mac.

You can create and export an Photos slideshow as a movie file without a 3rd party app.

However, you can't burn it to a video DVD disc without using a video DVD authoring application. The reason for saving as a disk image first is to save on discs when the encoding and burning goes bad and it does more frequently than I'd like. Also, the Blu-Ray player must be able to play regular video DVDs.

I don't believe you can use Apple Music for slideshows that are exported out of Photos unless they are tracks you owned and uploaded to Apple Music yourself. I don't use Apple Music so don't have any first and info on that.

I suspect I could then delete the 900 images from Photos after creating the slideshow?

What do you expect to achieve by doing that? If you ever want to use those images again for any other reason your won't be able to get them back.

1 - Photos comes with the system and is located in the Applications folder.

2 - you can play music and use it in slideshows but if it's DRM protected you won't be able to export the slideshow with it.