You can look at the number of games Thomas played at first and DH and ignore the fact that he didn't put up his Hall of Fame numbers as a DH. He played a lot of games at DH when he was hanging on after leaving the White Sox. Even his 2006 and 2007 seasons with the A's and Jays didn't come close to comparing with his average White Sox years, when he played more games at first through the 20th century.
The numbers he put up reflect that he was better offensively as a first baseman than a DH. If you look at the numbers he put up as a DH (which has been cited to me as a reason Thomas doesn't deserve to be in the Hall of Fame, silly enough, but I once dealt with someone 20 years younger who insisted Aparicio doesn't belong in the Hall of Fame), Thomas wasn't a Hall of Fame DH.