Apple Photo good alternative ?

How is the external disk that you have your library on formatted? It MUST be formatted either APFS or OS X Extended (journaled) and, preferably with Ownership set to be ignored;

Also how is the external drive connected to your Mac?

How big is the library and how big is the EHD? How much free space is on both your boot drive and EHD?

Since you have the latest in Macs you might consider an external SSD to house and run your library from. I use a setup like this:

However, the Thunderbolt version is about $16 and is faster than what I have:

The bare OWC SSDs are inexpensive and can be swapped out easily. I have 4 adaptors and 9 bare SSDs. Since I don't have the newer iMacs I don't need the latest Thunderbolt capability. I still get around 400 MB/s speed.

With the proper case that speed can be even faster but more expensive. I even boot from one when testing beta systems.

The iPhoto app let you put your personal interpersonaland your work on different systems and allowed you to share them as appropriate. Photo on the other hand doesn't share and has taken to crashing when you have more than 30 years worth of highly personal and irreplaceable photos and not allowing me to use my personal pictures without putting them in your "cloud".

Is it better now, after nearly a year? My experience has been, that Photos on Sonoma has been somewhat lagging during the first month after the system upgrade, but that improved quickly once Photos had finished the scanning of the Photos Library in the background, and after I had removed all incompatible older videos, images in PDF format, images saved from the web with the filename extension .webp

Older legacy items, migrated from iPhoto or Aperture, in a format that is no longer supported in the more recent versions of macOS since Catalina, can cause Photos to waste its time trying over and over again to analyse items that cannot be processed.Such items should have been converted to a more compatile format before the upgrade from macOS 10.14 to macOS 10.15, when Apple removed the the 32-bit iLive frameworks from the system.

DarBe75 wrote:

HDD is a mechanical Disk (not SSD) formatted in APFS

Spinning HDDs should be formatted as MacOS Extended (GUID) because APFS is VERY slow on them (the bits are scattered on the spinning plate so the seek times are long and longer -- SSD does not care about this).

The iPhoto app let you put your personal interpersonaland your work on different systems and allowed you to share them as appropriate.

So does Photos, and in pretty much the same ways.

Photo on the other hand doesn't share

Yes it does. And in morays than iPhoto ever did. Not sure why you think it doesn't.

has taken to crashing when you have more than 30 years worth of highly personal and irreplaceable photos

Pretty sure the age of your images has no relation to a crashing issue. Maybe you should explore that and see if you can solve it.

not allowing me to use my personal pictures without putting them in your "cloud".

This also is not true. Using iCloud is not at all necessary for using Photos.

LRC will migrate a Photos Library if you rename it like an iPhoto Library.

One worth considering: Mylio. More user friendly than LRC and excellent for sharing across devices.