Jenn, I was away visiting my daughter and family when you posted your cake and lemon rose pictures. Saw them but didn’t get a chance to reply in the midst of the hubbub of a house with young ‘uns. Good job!!! Hope you’re pleased with how your skill is growing by leaps and bounds. The more you do, the more you learn. That’s what I love about baking.
About the bitterness, your intuition to maybe simmer the peel is a good one imo. I noticed when I candied more peel for mincemeat that Rose said to do that with the orange peel, though not necessarily the lemon. So I simmered the orange for 30 minutes and put the lemon pieces in the last 10 or 15. Can’t quite remember. Hedged my bets, in other words. It is thinner than the orange peel, and I didn’t want it to turn to mush.
The other trick is, of course, getting rid of every last bit of white pith that you can. That’s where the bitterness is. I got quite compulsive about it. Used a thin sharp knife - sort of a paring knife but slightly serrated. Held the peel with my left hand, laid the blade against the pith and bore down as I cut almost horizontally across each piece. Got most of the pith before simmering, and went at it again, but gently, after simmering and drying with paper towels. You want to see dimpled yellow peel on both sides of the piece, very little of that white pith. Worth it, I thought, for the improved taste and more delicate roses. Hope that makes sense.