If you do want to stick with electromechanical relays I suggest carefully reading the following application notes from TE Connectivity/Tyco - DC switching can be hell on contacts (and on SSD's too):
NOTE: these are not really zip files - they are just pdf files with .zip added because the board won't upload pdf. Just delete that .zip suffix.
TYCO also has a more extensive document, but you'll have to get it at their web site as it's too big to post here. It's titled "Automotive Relays Application Notes".
Next, do be aware that contactors have limited cycle life, especially if switched under load. Some high current DC relays (moving armature rather than solenoid) are available that will last longer IF you follow what's in those above application notes. For bi-directional motor control you'd need two SPDT relays wired in an H-bridge configuration.
In addition to hobby motor controllers that Burgerman has pointed to, if you are up to making up a board for it STM has a 30-Amp rated all-on-one-chip motor controller, the VNH3SP30-E. It's cheap and reliable, but certainly using it would be more work.