(long post so read more added)
ok so. i was looking through my victorian flower meanings book i have. and i found quite a lot of flowers that could work. so here's my list:
flowers doug could cough out (if you dont know hanahaki au, usually the flowers you cough out symbolize the relationship you have. in this case, it does that, but also the relationship as doug percieves it as, if that makes sense.):
-bluebell (represents humility and faith, i think this kinda represents parkzer in their relationship, and something doug admires about him)
-foxglove (represents secrets, obviously he's hiding hanahaki from parkzer)
-hyacinth ("please forgive me", doug feels as though he's going to ruin their friendship with his love)
-lvy (represents attachment, pretty self-explanatory)
-amaryllis (represents pride, he's proud to know parkzer and proud of parkzer)
-clematis (represents ingenuity, i think they both consider each other really intelligent)
-iris (represents wisdom and faith, see above(s).)
-pansy ("you occupy my thoughts", again self-explanatory)
-yarrow (cure for broken heart, doug blames himself for his feelings)
-apple blossom (represents preference, doug prefers being around parkzer)
-camellia ("longing for you")
-honeysuckle (devotion and affection, doug's devoted to parkzer and vice-versa, doug's also affectionate towards parkzer)
-monkshood (chivalry, idk i think they both would be very chivalrous)
-orchid (represents elegance and beauty, i feel like doug would find parkzer elegant)
-sweet pea ("thank you for a lovely time", maybe this starts appearing when hanahaki gets really extreme?)
-zinnia (everlasting friendship, doug wants the friendship to last so he's afraid to confess)
-gladiolus ("you pierce my heart", love hurts blah blah blah)
-hawthorn (means hope, doug hopes everything will be the same)
-marigold (grief, if you want to go down an angsty route)
-azalea (fragility, doug fears their friendship will break)
-daffodil (represents unrequited love, either actual or percieved by doug)
-snowdrop (consolation and hope, doug doesn't want to confess bc he fears the response)
-dahlia (means eternal love and commitment)
-tulip ("I declare my love for you")
-rue (regret, maybe if doug wants to confess but doesnt?)
-rosemary (rememberance and wisdom, parkzer traits)
-magnolia (dignity, also parkzer trait)
flowers parkzer could cough out (Some of the flowers on this list are more about how parkzer feels about their relationship than only doug, also how he feels about confessing. idk i wrote this at midnight-):
-columbine (foolishness, but like in an affectionate way towards doug if that makes sense)
-olive (means peace, i feel like parkzer would be more accepting towards a relationship between them, esp since it seems like doug is more affectionate so he has more reason to believe its mutual)
-protea (transformation, the transformation of their relationship throughout the years)
-crocus (cleverness and youthful glee, represents doug)
-edelweiss (courage and daring, also doug)
-heather (represents luck and protection, he feels lucky to know doug)
-laurel (glory, victory, success)
-lily of the valley ("return to happiness", i'd imagine he notices doug's declining mood if they both get hanahaki at the same time)
-mistletoe ("surmounting difficulties", hanahaki isnt fun)
-oak (bravery, parkzer sees doug as brave)
-queen anne's lace (represents sanctuary, he feels safe with doug)
-iris (valor and faith, he has faith in doug)
-buttercup ("you are radiant with charm", represents doug)
-cornflower (hope in love, again i think parkzer would be more hopeful that it's mutual)
-dogwood ("our love will overcome")
-cattail (peace and prosperity)
-jasmine (cheerfulness, represents doug)
repeats (parkzer could also have these but im not rewriting the definitions):
tulip, apple blossom, honeysuckle, bluebell, amaryllis, pansy, zinnia, snowdrop, dahlia.
there's probably more that could be mutual but i'm not writing them all down.
-amaryllis & clematis (be proud of your cleverness)
-apple blossom & pansy (thinking of them)
-buttercup & cowslip (newfound attraction)
-camellia & daffodil (longing for your unrequited love)
-snowdrop & mistletoe (endurance)
-sweet william & honeysuckle (commitment)
-violet & laurel (proud of your accomplishments)
-yarrow & protea (the tides will turn)
-laurel, oak, & edelweiss (bravery in new territory)
-monkshood & cowslip (admire your courage)
-myrtle & dahlia (one true love)
-oak, sweet william, & monkshood (someone you admire)
-snowdrop & carnation (broken heart)
-hawthorn & orange blossom (hope for requited love)
-hellebore & edelweiss (courage in the face of whats coming)
-cornflower & honeysuckle (true devotion)
-iris & clematis (admiration for ingenuity)
-jasmine & crocus (zest for life)
-ladyslipper & hawthorn (hope for a good outcome)
-chamomile & dogwood (love overcomes all obstacles)
-cornflower & sweet william (i will always be true)
-daffodil & clover (hope for change in unrequited love)
-dahlia & myrtle (love and devotion)
-fern & floxglove (secret love)
-gladiolus, anemone & daffodil (unrequited love)
-zinnia, apple blossom, pansy, and eucalyptus (friendship)
-blush rose, cornflower, sweet william, and honeysuckle (courting)