"Is it common for the records to be so poor for the Gulf Coast regiments at the end of the war?"

It is common for most Confederate units to have their last year's records lost. For example, my ggrandfather's
43rd Tennessee Infantry have complete muster rolls on record through June 1863. Post Vicksburg, July 4, 1863,
only one subsequent muster roll is on file, and then, for only 6 of 10 regiments on March 10, 1864 covering the
July/Aug, 1863 bi-monthly period. They served till May 5, 1865.

I just checked the Texas Confederate Pension index, neither Tom, nor his widow are recorded has having submitted.