You can use NCBI Datasets for this. Specifically, you can use the command line tool for this as shown below:
$ cat accs.txt
XM_006719056.3
NM_001347425.2
NM_000014.6
NM_001347424.2
NM_001347423.2
$ datasets download gene accession --inputfile accs.txt --exclude-gene --exclude-protein
Downloading: ncbi_dataset.zip 12.3kB done
$ unzip ncbi_dataset.zip
Archive: ncbi_dataset.zip
inflating: README.md
inflating: ncbi_dataset/data/rna.fna
inflating: ncbi_dataset/data/data_report.jsonl
inflating: ncbi_dataset/data/data_table.tsv
inflating: ncbi_dataset/data/dataset_catalog.json
$ grep -A1 '>' ncbi_dataset/data/rna.fna
>NM_001347423.2 A2M [organism=Homo sapiens] [GeneID=2] [transcript=2]
ATACAAGAGATGTGAGAAGCACCATAAAAGGCGTTGTGAGGAGTTGTGGGGGAGTGAGGGAGAGAAGAGG
--
>XM_006719056.3 A2M [organism=Homo sapiens] [GeneID=2] [transcript=X1]
ATAAAGCCCAGTTGCTTTGGGAAGTGTTTGGGACCAGATGGATTGTAGGGAGTAGGGTACAATACAGTCT
--
>NM_001347425.2 A2M [organism=Homo sapiens] [GeneID=2] [transcript=4]
GGGACCAGATGGATTGTAGGGAGTAGGGTACAATACAGTCTGTTCTCCTCCAGCTCCTTCTTTCTGCAAC
--
>NM_000014.6 A2M [organism=Homo sapiens] [GeneID=2] [transcript=1]
GGGACCAGATGGATTGTAGGGAGTAGGGTACAATACAGTCTGTTCTCCTCCAGCTCCTTCTTTCTGCAAC
--
>NM_001347424.2 A2M [organism=Homo sapiens] [GeneID=2] [transcript=3]
GGGACCAGATGGATTGTAGGGAGTAGGGTACAATACAGTCTGTTCTCCTCCAGCTCCTTCTTTCTGCAAC
The data_table.tsv and the data_report.jsonl files include additional useful information that can be parsed, if needed.
Not that I am aware, but you should be able to match the biomaRt output back to the original data via simple functions.
With regard to a FASTA transcriptome, I may recommend those provided by GENCODE https://www.gencodegenes.org/