You are correct about one LAN subnet reaching the other like the Chromecast because Chromecast and friends are using mDNS multicast for discovery on the same subnet.
Like I said earlier, I keep my "old" wifi on the "old" router, so all the 43+ devices still rolling, my old wifi is still good, 802.11ac 3x3, same 3x3 on 2.4G, all casting devices still on old router. Your Linksys like I said it is a very good router.
My only concert when I do the double-nat was my VPN server, however it is working with port-forwarding.
The number of ethernet ports on Helix is ridiculous, i.e. 2, so no wire device goes there beside old router.
The only reason switching to the new Helix is to have a better package deal with videotron promotion.
I looked at the wifi on the Helix, on 5G it is a 4x4 802.11ac wave1 Quantena chipset, but who cares about 4x4 because there are no clients devices that support 4x4, and on 2.4G it is 2x2 Broadcom, not the best chipset, when I worked at HP/Aruba Networks, Broadcom chip is used for our low end access point. Your router is a Qualcomm chipset, that is "la creme de la creme" chipset so if I was you, I would keep using the old Linksys wifi and put the Helix wifi on a different SSID name and channel.
My biggest deception was the new Helix even though has phone ports(2) it is useless, not enabled, so I have 2 cables modem, new Helix for TV+internet and old one Harris for voice only. Please tell me it is bad.