It is so hard OP. Please don't blame yourself for any of this. You can see what an amazing, caring mother you are by the steps you are taking to try and fix this. Be kind to yourself.
I had a similar experience with my DD (not tongue tie so can't comment on that bit specifically but lost too much weight and wasn't getting enough from BFing alone despite feeding all the time). I followed a lot of the advice you've been given - always offering both boobs, doing compressions when feeding, getting the latch/position checked etc. I expressed (using a pump not a hakka) and gave top ups after each feed. I always felt like I didn't get out as much as I'd hoped and so felt like I was constantly either BFing or expressing. It was really hard.
I got ill and just couldn't face expressing one day so we ended up giving some pre-made formula as the top up (we used Hipp and the little bottles that come with the tip for ease). It was a game changer. She absolutely loved it and although I was a bit sad she seemed to prefer it to my milk, it was a relief to have a bit of a break from pumping. From then on, I did a combo of BFing, some expressed top ups and some formula top ups. I mainly just expressed once a day at the time I generally got my best results (around 1am) but didn't stress if I didn't get much as I knew we had the formula if needed.
Even when she was eventually on track with her weight and we were told I could reduce the top ups, we decided to stick with them as it was working well for us and we liked the flexibility we got from combi feeding, but I did stop expressing and just stuck to formula after a certain point. I eventually ended up moving some whole feeds to formula and just keeping some as BF as it just worked out better for us.
I ended up BFing until DD turned 1. I never thought that would be possible at the start. I'm glad a stuck with it and felt I ending my BFing journey on my terms in the end when I was ready for it to stop, but with hindsight I wish I wouldn't have put so much pressure on myself with it. Fed is absolutely the best for our kids, in whatever way works for them and you. It is really hard but you will get there in the end, it might just take a bit of trial and error. Take care, OP, you got this x