If you cast a spell with a bonus action, you can't cast a spell with a casting time of one action (PHB, p. 202):
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action
Does this apply to ongoing spells that provide an action after being cast?
A beam of brilliant light flashes out from your hand in a 5-foot-wide, 60-foot-long line. Each creature in the line must make a Constitution saving throw. On a failed save, a creature takes 6d8 radiant damage and is blinded until your next turn. On a successful save, it takes half as much damage and isn’t blinded by this spell. Undead and oozes have disadvantage on this saving throw.
You can create a new line of radiance as your action on any turn until the spell ends.
For the duration, a mote of brilliant radiance shines in your hand. It sheds bright light in a 30-foot radius and dim light for an additional 30 feet. This light is sunlight.
A beam of crackling, blue energy lances out toward a creature within range, forming a sustained arc of lightning between you and the target. Make a ranged spell attack against that creature. On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically
Am I wrong an interpreting this as allowing to cast a spell as a bonus action on the same turn as the actions provided by these spells?
- Turn 1: I cast sunbeam
- Turn 2: I cast hunter's mark, then use the action provided by sunbeam to create a new beam.
Best Answer
Yes, you can continue using the spell's effect
These two spells have casting time of 1 action, which means they are cast only once, unlike spells with longer casting time. This answer argues, that due to wording - which you emphasised in your question - subsequent activations are not casting (or re-casting, as linked question put it.) These actions don't have defined name (Attack, Disengage etc.) and you're allowed to take it while the spell is active.
Also worth noting that casting a bonus action spell doesn't stop Witch Bolt from functioning, unless that spell requires concentration. Examples:
I couldn't find a spell that allows activating it on following turns but doesn't require concentration.