In general, OpenWrt doesn't 'plan' to support any devices at all, device support happens if an interested developer or advanced user buys a new device for their needs and invests the time and effort needed to port it over to OpenWrt, which -depending on the device and its similarity to already supported ones- may take from a long and rainy weekend to (more likely) several weeks (months) to succeed. Quite a few devices will never quite make it, others may take several attempts from multiple parties building on top of each other's findings. If you want OpenWrt support for your device, the only reasonable choice is to pick a device that is already supported (at least in master/ snapshot), everything else is a gamble with varying chances of success (and no schedule attached); even devices that are looking close might still pose insurmountable issue during the last steps of their submission process.
Considering that this device has no chance of ever getting wireless support, no one sane would buy it with OpenWrt in mind and then spend the considerably efforts to get the basics supported.
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OpenWrt 21.02-SNAPSHOT, r15921-cd48473bb8
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root@OpenWrt:~# cat /proc/cpuinfo
processor : 0
model name : ARMv7 Processor rev 0 (v7l)
BogoMIPS : 1400.00
Features : half thumb fastmult edsp tls
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x3
CPU part : 0xc09
CPU revision : 0
processor : 1
model name : ARMv7 Processor rev 0 (v7l)
BogoMIPS : 1400.00
Features : half thumb fastmult edsp tls
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x3
CPU part : 0xc09
CPU revision : 0
Hardware : BCM5301X
Revision : 0000
Serial : 0000000000000000
root@OpenWrt:~#
It is very often that some devices are supported and we don't know about it. This is most often caused by manufacturers using the same chips across routers cause of lower development costs and mass supply discounts, between some router variants there are the same PCBs used and just some chips and their components are just unpopulated, this is all to reduce costs. As long as it's the same cpu and similar components around then they use the same or very similar boot-loader thus firmwares for one board work on another. Just look at what features does tomato have and what openwrt have and you'll find what is missing and then you'll just have to compile with the buildinfo of the RT-AC68U and add the drivers required.
The truth is that I thought I was going to brick the router.
I don't know anything about Linux, I'm a Windows user.
If the people who have the knowledge to do what you recommend do not help me, I will continue to use the firmware of the RT-AC68U on the RT-AC1900P, the only problem I found is that when I try to change the mac in wan interface, it doesn't work (Override MAC address).
Run some tests, put it through its paces, check if anything weird pops up in the logs, and if not, be happy that you have OpenWrt running on it. Like @slh explained, the chance of it seeing a dedicated image are rather slim. You might have lucked into a clone being supported already.
