Hello, GM Support main here.
My hero pool is 7 of the 9 heroes (No Mercy or Lifeweaver), however the vast majority of the time I will try and force either Bap or Brig, and then change when necessary.
The 2 biggest things I consider when choosing who to play is the other support and the tank, however the other support pick takes priority most of the time. Generally speaking I’ve never considered the DPS choice because on this patch and this meta, they are generally the ones expected to be the most flexible/counterpick, or will likely pick a sniper (Hanzo/Widow) that don’t require much help to be effective.
My general rule of thumb is if the other support is a “Main healer” such as Ana, Bap or Moira (Someone who could potentially solo heal if required), I will go Brig and be their permapeel and deny the space so the backline can’t be dived as easily, while also offering a defensive ult with Rally (Post-rework), which happens a lot in GM as Ana-Brig is a very popular comp atm. In any other case I will pick Bap, that way regardless of the hero picks there’s guaranteed to be enough utility to at least not throw a team fight, and enough damage to pick someone off out of position. The only exception is if the other tank is diffing enough I feel it would be better to go Zen for the Discord damage, or if my tank is Wrecking Ball, as good Wrecking Balls need virtually 0 healing from supports, and any combination of supports (Apart from Lucio-Zen, which is almost always just throwing or trolling) can sustain the DPS. Stacking double main heal is generally a bit more frowned upon at high ELO because Support =/= Healer, you’re eating away at each other’s ult charge, and after like Gold ELO you can’t get away with just healing, you gotta get in there and learn to find ways to contribute to fights offensively (Which is why Zen is considered the best support in the game at high ELO).
At low ELO you can also just lock Ana, Moira or Bap (The damage heavy supports that can also get decent burst healing in when teammates are low) and frag your way out, not relying on doing a lot of healing. You may hear the phrase “Damage is healing”, which sounds daft and counter intuitive but is true. The way to win the typical team fight is to bring the other team’s HP to zero, or low enough they disengage and you can progress the objective. Low ELO players generally assume that you do this by having supports healbot to keep the team’s total HP up, and for a short burst this may buy you some time, but if you were to pick a support that can also get the team’s damage numbers up either by doing damage yourself or by amplifying someone else (Think Mercy’s blue beam or Zen discord), you’re going to do more damage than what can be out-healed, and thus win via out resourcing the other team’s backline.
Your low ELO teammates will probably feed whatever support you choose to play, they’re stuck in these games just as much as you are, and this feeding is what’s holding them back, not a perceived lack of healing, you can either blow all your utility trying and usually failing to keep them alive because let’s face it, if they think they’re getting healed, they’re just going to feed even harder thinking they have the green light, or alternatively, you could try and help secure kills and damage before your teammates get low enough they start to panic.
TL;DR: Healbots get hardstruck, don’t let low ELO players tell you to just heal, they’re wrong, getting kills is how you win and climb faster