to get lots of caffeine you need to infuse your tea at least 3 min, then drink lots of it
(for me about 3 cups totaling ~ 1L i'd get something... but its a different feeling compared to coffee)
tea bags might have more caffeine, recommended ones are Yorkshire Gold by Taylors of Harrogate, PG Tips by Brooke Bond (both UK brands, might find them in some supermarkets, or British import stores, or online.)
not sure how it will differ in your health issue since the main 'active' ingredient is still caffeine... tea can also interfere with medications...
if you want a strong taste+astringency, indian teas give you that and lots of it especially if you use lots of tea or infuse for a long time, Assam in teabag or looseleaf or CTC (CTC = little pieces, can be found at some Indian markets very cheap)