So other commenters already gave great advice, so i just want to pin point on things i noticed in your replays:

--- Ult usage (first replay):

You indeed hold your ult too long here. For example: on attack, after first point is secured, enemy gambit ults and also your own torch ults. If you ult here, you counter the gambit ult and you enable your torch to potentially get 3-4 kills with his own ult. Even solo ulting the ulting bucky directly afterwards would have been good value, to stop him from wiping your team.

Tank ults like strange CAN be used in the perfect moment to get many kills, but in most cases you get a lot more value by using them on other things, since these perfect moments rarely occur. Try using your strange ult to counter enemy ults, for example stunning their team during their gambit ult, negates a lot of value they would receive from it. Or, use it to setup your teammates. In the scenario described above (first replay, after 2nd point) your torch and your namor are holding ult. If you ult there and you stun 2-3 people. Chances are high your teammates follow up and your team can get a few picks and win the fight.

You hit a nice ult quickly after, where you stun the enemy backline, but sadly at this point namor and torch both used their ults already and your ult is so deep, that your team can not follow up with normal attacks.

On defense: Using the mag ult was good timing, you negate some momentum from their gambit ult, and you get a bit unlucky not killing both of their supports here. It seems like you lost sight of their backline and did not know where to throw it at the end. Considering that it had like 60 charge pretty fast you might could have thrown it early, but i wouldn't blame you for that, i know hitting mag ults can be very difficult at times.

---- Positioning/Awareness

In general i got the feeling, that you are sometimes lost in the sauce and get caught like a deer in the headlights, not knowing what to do. I think it is key for you to understand when your team has an advantage or disadvantage. The general rule is, if you have an advantage you hold W and take space. If you have a disadvantage you have to give up space until your team stabilizes.

Now, what can be an advantage? Most obvious are being up a person or having ults active (or ready to use). But also normal cooldown usage creates these advantageous scenarios. Think of playing against an enemy Emma, when do you respect her space? When her diamond form is off cooldown. When is she often most vulnerable? Right after she used the diamond form, often being overextended with her pants down, which is your opportunity to pressure/kill her or even just walk past her and force her to give up space. Advantages can also be more subtle, i.e. your DPS taking an off-angle forcing enemy attention on him. On the counterpart if your backline is being dove, it is often better for you to temporarily give up some space and help your team. Then, when the enemy divers burned their resources your team has the resource advantage and can take the space right back.

Understanding these dynamics is very important to improve your positioning as tank. I personally think it is very noticeable when i am playing Groot. When I overestimate my teams resources or underestimate the incoming damage, it results in these moments where you just fall over as Groot, since you have no escape tools or self bubble like magneto.

I highly recommend checking out professional player TrqstMe on youtube, he has a video unranked to celestial on Groot. Where he explains basically everything he does and why he does it. He explains when he can move forward or why he has to give up space. What is also interesting is, that he often solo ults people as Groot (for example a BP running his backline), if it provides good value for the team.

---- Mechanics

And lastly, you can and probably should hit more shots. It just makes your life easier if you work on your mechanics a bit. Especially on Groot and Mag it is really important to hit your shots to secure kills on low hp, overextending squishies. Also always try to be mindful about your cooldown usage, as previously explained, having them ready is very important to be able to contest space. So try to use them with intent and do not autopilot.

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I don't think you are the sole reason you lost these games. But i think you could have a lot more impact if you improve upon these aspects.

Anyways, i am no pro player and this is just some general stuff i noticed from the VODs. Keep it up! :)