This is a learning exercise for myself, but in the event this is of interest to others....
In short: after a few edits, it seems to mostly work (ipv6, ath10k-ct, 2.4 and 5 GHz wifi, guest networks, strongswan, a few firewall rules for a custom etherwake server wol script).
root@OpenWrt:~# uname -a Linux OpenWrt 4.19.52 #0 SMP Thu Jun 20 11:23:37 2019 armv7l GNU/Linux root@OpenWrt:~# uptime 21:09:10 up 5:07, load average: 0.00, 0.03, 0.00 root@OpenWrt:~# df -h Filesystem Size Used Available Use% Mounted on /dev/root 3.5M 3.5M 0 100% /rom tmpfs 233.1M 1.1M 232.0M 0% /tmp /dev/ubi0_1 78.7M 19.7M 55.0M 26% /overlay overlayfs:/overlay 78.7M 19.7M 55.0M 26% / tmpfs 512.0K 0 512.0K 0% /dev
I started with master and cherry picked the ipq806x kernel 4.19 commit from here. This commit does not (yet) have the changes to qcom-ipq8064-r7500v2.dts to expand the overlay (see PR 1922). I made the overlay expansion change to the 4.19 dts file since I want to upgrade/downgrade via sysupgrade from luci.
In the absence of other knowledge about usb and kernel 4.19 on the r7500v2, I edited the qcom-ipq8064.dtsi and qcom-ipq8064-r7500v2.dts (only the usb entries) following the discussion about usb on r7800 and 4.19 (see this response) from @Ansuel.
After some help from @Ansuel (see posts below), USB is now working.
There are a few "failures" in dmesg that don't show up in 4.14 related to cpu idle and temperature sensors (see below - collectd-mod-thermal does not display temperatures).
After make distclean, make menuconfig (I made no changes to my usual custom .config), make defconfig, and make kernel_menuconfig, i ran into this error applying a 4.19 kernel patch. Following @slh's advice in the next post of this thread and directly editing the patch to match that in 4.14 (I only needed to change a few lines) resolved the issue and I was able to build and sysupgrade the image without any trouble.
Helpful instructions (r7800) for tty serial tftp booting initramfs images for testing (i.e. avoid flashing) here. See this post below for a worked example on the r7500v2. For reference, the generic serial flashing guide. Example for the ipq8064 based NEC Aterm WG2600HP).