How does the remaining validity of my passport affect my LTR WP Visa eligibility in Thailand?

I renewed my passport 2 years after getting my pensioner LTR. You just go to the Bangkok BOI office and they will transfer your LTR visa to your new passport. There is no fee. Your new passport will be stamped for the remainder of your 5 years

The BOI office is in One Bangkok. There is an immigration office across the hall.

I'll speak for picking up the visa in Thailand and getting the visa inside of your passport first.

Once you get the final approval you choose picking up in Bangkok as your option then you make an appointment to go to the BOI office in One Bangkok. There you'll be taken to the immigration office, pay the 50,000 baht, and you'll receive a 10-year visa inside of your passport. Next to that 10-year visa, you'll receive your "entry" visa, which in your case will be until the end of your passport validity.

When you get a new passport, you take it to the BOI office and they will get your visa moved over to the new passport, where you'll receive the same 10-year visa stamp, and then the other stamp which will go until the end of your initial 5 years. Note that this can ONLY be done in Bangkok at the BOI office and associated immigration there.

If you instead choose to get your visa from a Thai embassy, your visa will not be in your passport but instead will be a printable PDF. When you enter Thailand you'll be stamped in until the end of your passport. Once you get a new passport, the process should be the same, go to BOI with the old and new passport and your visa.

The only difference with the printed one from the embassy is that your initial 5 years doesn't start until you enter Thailand.

You need to be very careful about how you do this because the BOI has sent MANY warnings about losing your LTR if done incorrectly. You either need to get a new passport while you're already in Thailand and then go to the BOI to get everything moved, or if you get a new passport outside of Thailand you MUST be back inside of Thailand before your original passport and the permission to stat that you received expires. That's because you need to be stamped back in with an LTR and if your original stay expires you cannot be stamped back in on it and you will lose the LTR.