Dakota, whatever you do, don't buy an iMac with a hard drive. Get an SSD - the biggest you can afford. Hard drives are going the way of the dodo. You can always use an external drive if you need more storage for things like photos or music or video. You cannot upgrade the drive yourself later - or not easily, anyway. That's worth getting right when you order.
Upgrade the RAM yourself when you can - 8GB is kind of minimal these days. With the 27" iMac it's easy to do yourself, and generally cheaper than buying it through Apple.
As for your backup - get a couple of external drives and use one for Time Machine and the other for a cloned backup using SuperDuper or Carbon Copy Cloner. Set it to back up automatically. I have a RAID backup for my iMac now - it's USB-C, plus a TimeMachine backup (on a 4 TB external - Western Digital Black, in a Rosewill USB 3 enclosure). You should be using USB-C going forward, since both new Macs should support it. I have several Samsung T5 external SSDs for my photos; they're wonderful. Fast and very compact. My entire Lightroom library lives on the 2TB one.
I second the recommendation to order from the Apple refurb store. I've either done that or used my kids' university discounts (which are available to parents!) to order all Macs lately. However, at the moment, there are all of two 27" iMacs on offer, so not much in the way of choice! B&H are very good - I used to get all my camera equipment from them when I was living in the US.
I'm assuming this is the one you got earlier?
https://everymac.com/systems/apple/imac/specs/imac-core-i5-3.7-27-inch-aluminum-retina-5k-early-2019-specs.htmlIf you go for an internal SSD (which I think you should) then you will have to pay a little more to get any capacity. The same model iMac you have, but with a 1TB SSD is $2599 at B&H:
https://www.bhphotovideo.com/c/product/1468645-REG/apple_z0vt_mrr12_03_27_imac_with_retina.htmlThe exact same one you have is on sale right now at MacSales (another excellent dealer, who I regularly dealt with when living in the US):
https://eshop.macsales.com/configure-my-mac/UACM3F7ES2130XIAND for $2399, you can order a refurb one with a 2GB SSD. That is an amazing deal, given that the list is $3699. One year warranty too.
https://eshop.macsales.com/configure-my-mac/UACM3FP7XX230GI (that one would get my vote!)
For a router - we've been very happy with our Asus AC-5300. It's a beast, and we've had zero issues with it. Great range. There is a newer version of it: Asus ROG Rapture GT-AX11000 - but it's rather expensive. (ours wasn't cheap 3 years ago either) There are a lot of good reviews for the more reasonably priced Netgear Nighthawk AC1750 R6700.