Hello!
To change the payload to SeaBIOS instead of coreinfo, skip step 4, and in step 5 under "Select the payload" you would select SeaBIOS in the "Add a payload" menu, and then exit and build as usual. Then when you run qemu in step 8, you should see SeaBIOS instead of coreinfo.
Once you are familiar with that you could try building coreboot for your Thinkpad. To do that, first run "make distclean". Then, run "make menuconfig" (like step 5 in the part1 tutorial on
doc.coreboot.org) and select Lenovo as the "Mainboard vendor", and the appropriate model in the "Mainboard model" menu. SeaBIOS should be the default payload, but you can check that in the Payloads menu. That will produce an image that should work on your hardware. I won't go into the details about flashing it, as I am not familiar with the exact process for your laptops and there are plenty of flashing tutorials available on the internet.
Also, feel free to join us on IRC (#coreboot on libera.chat). There you can more directly communicate with the coreboot community and likely will get answers sooner. If you aren't familiar with that, the easiest way is probably by going to
https://web.libera.chat/, entering a nickname in the "Nick" box and "#coreboot" in the "Channel" box. You don't need to register, but you can if you would like to reserve your nickname.
Cheers,
Nicholas