Closest match style wise would probably be Shostakovich, start with popular middle symphonies 5-10. Best to mix and match here but if you want one name I go with Jarvi/Chandos
Later Sibelius would also be sympathetic, the early 1,2 symphonies are romantic works similar to Tchaikovsky symphonies, Sibelius 3 is transitional and 4-6 would be close to Mahler 3,5.......the Maazel/London is an excellent Sibelius set.
Also along those line try Nielson 4,5,6 many good 2CD sets like Bloomstedt/London
Strauss tone poems......sure why not, unless you are seeking only symphonies
Bruckner is really a Wagner disciple, building layers of sound that ebb and flow like tidal waves, erecting massive "cathedrals of sound" Style wise really different than Mahler but Bruckner has works of great beauty that every classical fan must own......perhaps a common theme that filters through both composer's work is religious element.
Obvious comment but collect all Mahler symphonies if you don't already own.