Linden could work for all genders I think. I'm not sure what my favourite tree name for boys is.... maybe
Rowan? You never see
Rowan on boys. Alder is handsome too. And I love Hawthorn.
Mega handsome.
Tacy is interesting. It seems a bit wispy, just a scrap of a thing. I agree it sounds modern and made-up, ut it's neat that it's not. I'd like it as a nn for
Tacita I think.