20000 slides, do the math at 2 minutes each. It would take half a year with the Epson scanner if you worked at it for 7 hrs a day 5 days a week.
You should consider an automatic back up program to clone your photo files to your external backup drive if this is how you want to implement your scheme (I would also recommend cloud backup of your external drive). I have been using SuperDuper successfully for four years. If you are running a mac, are you also backing up these files with TimeMachine?
I had about 1000 slides done by ScanCafe. Not impressed with result. Subsequently acquired an Epson V600. Subsequent re-scans of the best slides showed significant improvement in the result.
Went through the Aperture to Lightroom transfer process. Be aware that your edits do not transfer unless you export your processed images and transfer them to the new software. If you do this, your loose the ability to re-edit your original photos which may be the OTC RAW images.
I use the Geotag app on my iPhone in conjunction with my Nikon D7500. It requires one extra step to tag all of the photos (I shoot only RAW) before import to Lightroom. This only takes a few minutes.
Vello corded for both my Nikon and Sony cameras.
I use Google Blogger. It is easy to use and supports embedded photos. You can create a separate blog for each trip as opposed to WordPress that is one continuous blog. I paste a link to the blog in my Facebook update posts and friends can click on it to go directly to the blog.
Before picking a provider, think about the restore scenario. If you have a large volume of photos, video and data (I have about 3Tb), how long is that going to take to restore via the internet? Backblaze will provide your backup on an external drive for restoration which would reduce the time to hours rather than days.
The 13" MacBook Pro similar to the one described above has worked well for me over the past five years for travel to Europe and Australia /NZ. Not much bigger than an iPad and configured with LR/PS allows full editing when on extended trips. Just export and merge your MacBook LR library with your iMac LR when you return home and all of your editing work is there. I carry enough memory cards to back up the original images until I get home and have everything backed up to the cloud. (Backblaze)
Just sold a D7000 body on Facebook Marketplace. $250 cash.
Try the smartphone app Geotag Photos 2 and add location to your photos taken with any non GPS equipped camera.
Chromecast and a free program called Plex Media Server
Another vote for Backblaze. Don't know of any "free" cloud service that will back up more than about 20Gb.