My startup disk is a 256G internal SSD of a Mac mini which is full of apps and system data that can not be moved, and although there are more than enough free space on other external connected 4T SSD and 32T disk array, while I tried to render a sequence of 4000 raw photo and save it to the external disk, LRT fill up my startup disk with intermediate files and failed, please advise is there a work around?

BTW, I think let the user specify where the temp/cache folder is a better solution, for me , the startup disk is so small and since the large amount of data written to it by LRT, the SSD Life time left decline stunning rapidly, I think it will destroy my startup disk very soon.

No SSD gets destroyed by normal working on it. This just doesn't happen.
LRTimelapse uses the fastest drive (which is usually the drive the system's temp folder is located on) in order to allow for the fastest processing speed.
If you have such a small SSD internally, you can set the temp directory that LRTimelapse uses to another drive, but make sure that it's a fast one. You can define a different temp folder in the lrtimelapse settings under "Expert Settings".

Thanks for your reply, that's a great save.

Attached please fine the SSD Lifetime Left screen shot, it's 93% the day before yesterday, I rendered about 20000 raw photos in these 3 days before I encountered the out of space problem, and now it was down to 87% .

That number for the "Lifetime" is bogus. It's a rough estimation, that dropped most likely came because your ssd ran full. SSDs don't like that very much. I assume it will go up again over the next days or weeks. In any case, no one knows when an SSD will fail. You can ignore such numbers.

But who would use a computer with a 256 GB ssd for timelapse work on ten thousands of raw images?

Do yourself a favor and get yourself a large and fast SSD and work on that. LRT mostly writes proxys and previews to hidden subdirectories inside the images folders, that's why it matters that those drives are fast. So if your images are on the large and fast ssd, it's fine.

The temp folder will mostly be used to render the video files before copying them to the output folder.
Therefore set the temp folder to the big SSD too.