Becoming SEN stations would lose them the racing radio exception to local content - so they’d need to meet the local content quotas - which might be enough for them not to bother, and just do what the other SEN Track stations do and just push the limits of non-racing content.
3BT isn’t owned by RSN though (it shares a PO Box with Capital Radio, but not sure if directly owned by them), only 3SR in Shepparton is owned in common with RSN/3UZ.
SEN pick up a handy chunk of DAB space with this too - RSN bought extra at the excess capacity auction - so I would expect SEN to be able to sublease some of that for some extra cash, as it would likely be well beyond their needs, as I don’t think there’s ever much content that ends up on the RSN Xtra stations that couldn’t be accommodated on SEN Track or Fanatic.