Play the game that you like to play.
Play the game that you like to play.
"Given the opportunity, players will optimize the fun out of a game." - Soren Johnson (2011), source: https://www.designer-notes.com/?p=369
How much you need to heal and how much you get to DPS as a healer really depends on the group. If your tank pulls a lot, they'll need more healing. If they pull more than they can handle, they'll need more healing. If they pull more than the DPS can handle, they may need full-time healing with cooldown-usage on your side. I once got called out that I had to 'focus less on DPS' when a tank died after pulling way too much, the DPS couldn't handle it and I was out of healing cooldowns without being able to even contribute to DPS. This goes to show that how you approach any given situation depends on the dynamic of the party and the current situation.
In general, I'd personally recommend to keep healing as your top priority. When there's nothing to heal for a bit, fill up the time with some DoTs and damaging spells. "Everyone" (read: sarcasm) may want you to put DPS first, but that's purely because of the quote I put above.
At the start of the game, you'll be eased into the game's mechanics and approach at a comfortable pace, but this ramps up with noticeable difficulty as you go on. With Shadowbringers and its (instanced) content, the game has shifted to a more healing-focused approach for healers. You'll be DPS'ing when you get some downtime, but as a healer I find that a vast majority of my time is spent healing (partly for making up for mistakes in the party. They simply happen).
What I would advice you is simple: try the various healers, find one with a toolkit that suits your gameplay, keep healing as your primary concern and keep DPS a secondary priority. You are, after all, a healer.