Syncing with Smart Previews

"My catalog has 170,000 photos. Would that be too many to sync then?"

It is theoretically possible, but why? You would need to place all those photos in Classic Standard Collections, and a lot of bandwidth time for Lightroom to upload the Previews to the Cloud. (Note: they are "Previews"- NOT image files.). And do you really need to have all those photos available to edit when 'mobile'?

"do I need to use smart previews at all?"

No, you probably don't need to. Lightroom-Classic works with the original  'local' photo files, on connected internal or external drives. Smart Previews were developed as a means to allow editing when an external drive with the photos was disconnected and original files were not available to Lightroom-Classic. At a time some found that Lightroom worked faster if set to work with only with the Smart Previews (a Preference setting).

Personally, I Sync a thousand or so photos that I added to my Lr-Classic Collections. They are available in the Cloud for me to view, edit, and share with family on-line, on my laptop in Lightroom (the Cloud version 2.4.1) and my iPhone (Lightroom mobile). I have never had to manually create or use Smart Previews- the "Sync" of Previews is an automatic process, and definitely NOT a means of backing up original photos!

Curious if it's possible to take the smart previews I've created in LR Classic

You do not need to make Smart Previews, you do not take the smart previews anywhere- it is done automatically by Lr-Classic. (Do NOT consider Smart Previews as Image files!)

and edit them/view them on LR/online cloud,

Yes, Open Lightroom (cloud version) on a Laptop, or Lr-mobile on a phone, and edit.

and also if there's a way to sync any changes later on back to the LR Classic library?

This happens automatically.

The method involves-

1. In Lr-Classic Create a Standard Collection of photos that you want to edit remotely,

2. Mark this Collection to Sync

3. Set Lightroom-Classic to Sync (beside the top Identity Plate)

4. Remotely edit in Lightroom on a laptop or iphone.

https://helpx.adobe.com/au/lightroom-classic/help/lightroom-mobile-desktop-features.html#AccesssyncedLightroomClassicCCphotosinAdobeCreativeCloudmobileapp