New iMac setup seems endless with Time Machine backup including big Photos archive

I am asking this on behalf of my artist wife, as I am inexperienced in Mac.

Her 2012 iMac 27" just died. No boot or screen response on attempting to start.

She was running Time Machine backups to external USB HDD.

We bought a new iMac from another store at minimum spec (8-core CPU, 256 GB ) and on setting up used the Time Machine Restore option.

The Time Machine backup of her username was over 600GB.

That failed because the disk was too small so we had to return it and buy one (this time from Apple) with 1 TB. This time its the 10-core CPU and four USB-C ports.

Now we have had the setup/Time Machine restore running first time was for two days, and the progress screen shows 737,459 files loaded - and each time we check the number of files loaded is the same, but estimated time remaining gets BIGGER. After two days, estimated time remaining increased to over 12 hours.

She called Apple support. On their advice we restarted the process.

Now we are 19 hours into this second attempt. There are 737,459 files loaded and 2 h 24 m said to be remaining (still rising), and transfer speed stays at 127.7 MB/s.

Her Apple Photos archive includes a truly massive file after 12 years filling up, perhaps with multiple layered images or incrementals in Time Machine. Perhaps this gives cause for a horrific restore time for one file.

By searching this support community, I see that Photos archive files are linked and layered and must be restored with internal pointers or links intact. Does just letting this run inevitably not work, or will it come good in a day or two?

For her art work she long-term needs the whole lot back, but firstly she needs a working computer.

We are four days in without yet seeing the desktop of her new machine. What should we do?