well, as i said i only Play him here and there but i think i can put up enough Knowledge to have a fair judgement over how i personally would Build him and, who knows, maybe even give a helpful recommendation!
so as for thresh, he is imo the one you can most possibly compare to janna here in Terms of Build because both of them can work with pretty much the same, more or less, the difference is thresh can utilize tank items like sunfires and randuins better while janna can profit from some ap like a morellos or athenes.
as for thresh itself i Build depending on my comp vs their comp entirely, doesnt matter what adc i have in a specific case.
do both teamcomps possess a lot of brawling potential or are otherwise focused on Extended fighting via onhit or lots of tanks then i go with standart items like a
which the latter 2 can be optionally replaced with
due to the reason that i want to mostly focus with a lot of defence while also stacking CDR for my peel so my E is up most often and i can disposition their ranks. hereby i heavily focus on the midgame and while my earlygame is rather unforgiving for mistakes in trades or stupid all Outs, it Spikes strongly for the later phases and helps my team Overall get past the 30 minute mark with positive results.
if their team has a lot of mobility or i Need to snowball the ever living shit out of the laning phase incase they have something like
combo or a
and
in the enemy ranks then i exchange
with a
and rush
first so i can maximize CDR and cooldowns while also maxing E first instead of Q so i can make the most out of early trades by slapping them harder with my auto attacks
with thresh you always have the question of are you gonna peel for offense with Snowballing and dispositioning for an provoking attempt to bait mistakes, or are you gonna peel for defensive purposes, disposition to aid your team in having a superior trade and cut off their ranks from assisting each others, per say split the supp from their adc when they have an heavily defensive support like
so depending on that you have to considere also going for a
to further slow their comps down if you dive in so they cannot Reposition as quickly.
it is ideal for me because i Play thresh as this kinda guy who does not lock down one Person, who does not peel or hook like madlife, but personally focus on simply disorienting their ranks so my team can do most of the job, so you have to probably stack some tanky stuff and maybe even consider some bonus heals and shields.
having said that my go-to standart Build for thresh is
(which late into the game get changed to merc boots since i buy them early on to maybe roam mid here and there) and
OR 
now many support mains will raise a brow and tell me "why not a ruby sightstone?" reason here is beyond simple. the biggest cooldown here is
with an 60 second cooldown i believe, redemption and talisman have a 45 second cooldown which ticks once the proc is fully completed, which makes the item cooldowns so short that already in the early they are most likely present and ready to use for every fight that happens ecspecially when it then transcends into the lategame.
Assuming you Build randuins you are already in the lategame so it's comparably high cooldown with 90 seconds (again i think, not sure thoe) will be rather irrelevant because one fight will pretty much decide win or lose unless you as the thresh supp are the last man standing, and Knights vow you are likely not going to change fast so it's 45 second cooldown to reactivate the bonding becomes almost entirely nonexistent. i combine low item cooldowns that can all-round serve me well while i also stack the hell out of low cd for my skills to maximize my peel and my depositioning of the enemy comps, which in Terms makes me into this sort of sabotaging infiltrator rather than a purely peel or defence type of guy.
janna for the most part is similar except you can switch out
with either a
or
and the last item make the second of These two, whichever you choose but she and thresh in my opinion have 4 items in common, to which talisman is optional and you can possibly go for a
instead, both work on her honestly.
as a thresh newbie who just Plays him here and there when i get autofilled as a support this is by far not the most optimal recommended guide but it is what works best for me and i hope this gave you some answers