Oh you're so, so, so welcome! And I can completely understand your anxiety! I've had contrast dye 3 times (today included) and it's really no problem. I promise, you really won't notice it. You feel a tiny breeze of a cold rush going into the vein of your arm as they put it in, but after that, nothing. I am always afraid though... every single time. 
And I can't stand MRIs! Just close your eyes before they roll you into the actual tube. Tell them you'll want to close your eyes and make sure they don't roll you in before your eyes are shut. This really, really, really helps to avoid claustrophobia in the machine. Just keep your eyes closed and daydream about all sorts of things. You can listen to music as well. MRIs are a little bit crazy and loud, but they are amazing! It's the gold-standard diagnostic equipment for sure. You'll definitely get answers.
I do wonder if the covid may have had some sort of impact on your heart. It is a vascular disease, Covid, and there is a correlation between Covid and heart implications. But try not to fear. I live with mild heart failure and it doesn't significantly reduce my life expectancy. Not that you have heart failure at all. But you can still live a long, good quality, happy, healthy life with heart 'stuff'. I still ride a bike everywhere and walk the dog daily. I have to be careful not to get my heart rate going too fast, but it doesn't stop me from enjoying things. Though, I can't ride rollercoasters. Amazing but true. Not that I'm riding many these days other than emotional ones.
I'm 49 now and I was diagnosed in my early 30s with long QTs (not sure if I have long QT syndrome itself but I was loosely monitored by cardiology for about 3 years then discharged).
I have an echo and ECG coming up as well, so hopefully, I'll get some long-awaited answers. When I was younger, they did no scans. I had an ECG and was diagnosed with long QTs back then. But I never had an MRI or Echo back then and I probably should have asked for one or the other. I've lived with bigeminy for which I've never gotten answers (I never pushed for any. It's really my own fault). It was the tear in my artery that's now got me so heavily plugged into cardiology again and it's also this very tear (called a SCAD) that has the consultants looking for Marfan's at this stage.
My cousin with Marfan's was diagnosed as a younger adult. She's in her 50s now and continues to do really, really well. She's a biochemist in Australia, living a healthy, happy life. Her Marfan's hasn't impacted her too much and hers isn't a mild case, so in a funny way, that is reassuring. So, take heart (literally!) in the fact that people with Marfan's, and other heart conditions, can and do live long, and very productive, happy lives.
I think we are incredibly fortunate to have some absolutely amazing heart centres here in the UK. Whatever is going on with your heart, you will get to the bottom of it and you will be well managed and looked after. 