Delete originals but keep Smart Previews?

Another vote for not deleting. You never know!

My recommendation would be (assuming your image folders are organised primarily by year, which is a widely used arrangement) to move the disk location for some older years onto another drive, while still keeping all those photos imported to your Catalog and (optionally) supported with Smart Previews..

Method would be: on separate drive volume (other internal drive, external drive, network attached storage, whatever) make a top level folder for ease of management. Inside this, working outside of Lrc, copy some older years-worth of image folders. For example, you might one by one drag in 2022, 2021, 2020 etc while leaving more recent years as they are. Now those are in both drives. Within the Catalog you would then go to (e.g.) 2020 and right-click, choose Update Folder Location, and then browse to the location of 2020 on this other drive. The Catalog now readdresses all of that. Once you are confident everything has gone suitably, and supported by file backup in the normal way, you can then remove the now-redundant instance of (say) "2020" that remains on your working drive. LrC is now looking elsewhere for all those 2020 source originals. If you were to now disconnect this other drive, the 2020 images would temporarily go offline but the (e.g.) 2024, images would still remain online. So this might be a yearly task to migrate the physical storage location of the next prior year. Perhaps at the end of 2024 you move 2022 across - proactively - or whatever else the size of your main drive dictates.

My 2TB drive is almost out of space; I want to stick to one catalog but rather not buy a larger drive yet.

One catalog can work with photos on more than one disk. Just put new photos on a different disk and don't bother with this idea of deleting anything.

Do not delete the original files from within LrC as this will also remove the smart previews. Instead, I would suggest that you connect one of your back up disks containing the same files, then link LrC to the back up drive. You should also check the integity of the disk, that there are no files missing, etc. The backup drive can then be disconnected and smart previews rechecked in offline mode. If all is well, then feel free to remove the files from 2TB SSD, but do so using the OS not LrC.