[Edited when I realized that the rollover deadline has already passed.]
"When I tried to transfer the funds to my Roth, I was told only $8000 could be deposited."
Had you done so by the 60-day rollover dealine you could have done an indirect rollover to the Roth IRA, not an ordinary Roth IRA contribution.
Because you separated from service in 2024, both 2024 and 2025 are RMD years. You are not permitted to roll over the amount of the distribution that represents either of these RMDs. Since the deadline to complete the 2024 RMD was April 1, 2025, it's not clear if this distribution included your 2024 RMD or if that was distributed separately prior to the total distribution of the 401(k).
"Do I have till December 31 of this year to take my RMD for this year?"
As explained above, you have already received your 2025 RMD as part of the total distribution and that portion was ineligible for rollover even if you had done a rollover by the deadline. The total distribution also included your 2024 RMD if that had not previously been satisfied. The 401(k) plan should have forced the distribution of the 2024 RMD by the April 1, 2025 deadline if that distribution had not already been made. (Perhaps the total distribution actually occurred by that date.)
