I don't think you can sysupgrade to 19.07 as from memory there was a partition size change, I think you will need to use a factory image via tftp to get it on 19.07 then sysupgrade will work from then onwards.
I currently use in Windows the TFTP GUI tool "tftpd64" from jounin, as the earlier tftp2 from dd-wrt stopped to work with R7800 about two years ago.
The OEM flash routine apparently has some timeout, which plays badly with some clients (with default settings).
It should make no difference. The u-boot sequence just reacts to the reset button press (or actually a release) during the "white flashes".
Ps. Somebody has apparently recently added the incorrect/unnecessary "at least 10 times" advice to the R7800 wiki page. I have written the original version some years ago (originally to the forum in 2016 and to the new wiki in 2018), and there is no requirement of "at least 10 times". The key is to keep the reset button pressed, wait until the orange flashing (OEM Factory reset mode) is over and the white flashing (TFTP mode) begins. Then release the reset button. And the router will be in the recovery TFTP mode waiting for you. There is no rush after that.
As seen from serial:
U-Boot 2012.07 [local,local] (Sep 03 2015 - 17:33:28)
U-boot 2012.07 dni1 V0.4 for DNI HW ID: 29764958 NOR flash 0MB; NAND flash 128MB; RAM 512MB; 1st Radio 4x4; 2nd Radio 4x4; Cascade
S17c init done
Factory Reset Mode
The Router is in TFTP Server Firmware Recovery mode NOW!
Listening on Port : 69, IP Address: 192.168.1.1 ...
Upgrade Mode Upgrade Mode
Ps. I edited the wiki a bit to clarify things again.