Vyvanse/Tyvense /Lisdexamfetamine works better when not taken for a few days. Why and Advice?

Background

I've been diagnosed with ADHD since September 2023 (by 2 different clinics) and I've tried a lot of different medications with different dosages; Ritalin showed no effect except bad headaches on 20mg and 40mg. Stratta was never in stock so I never tried it. I tried Tyvese up to 60mg which I thought at the time wasn't working because from continuous use there were no effects. Then I went with a new clinic as my previous psychiatrist wasn't the most helpful with telling me what to do next. The new clinic put me on Concerta starting at a low dose and up to 50mg I think? It didn't work at all and headaches again. This autumn I decided to go back on Tyvense on 30mg.

The problem

The effectiveness decreases on Tyvense depending on how and when I take it. For example, let's say I haven't taken it in a week and then start on a Monday, it works well that day. Then taking it on Tuesday, the effect is noticeably less. On Wednesday its even less effective and so on till Friday. Then I take a break on Saturday and Sunday. I start again on Monday, and it won't be as effective as the first Monday but still more effective than the mid-week where it's not as effective. I've been thinking about ways to address this issue because I'd really like to take it Monday through Friday and consistently feel the effects throughout the week. One idea I had was to vary the dosage, so starting with 30mg on Monday and Tuesday, then increasing to 40mg on Wednesday and Thursday. Has anyone tried something similar?

Things to note

I am female and overweight, (I know that this is not ideal for ADHD meds but I'm losing weight. My GP and psychiatrist are aware and are happy to continue with the medication. I am on Sertraline 50mg and Yaz. I have an eating disorder ARFID and I try to take supplements to make up for the loss of nutrition. Any advice I'm given will be discussed with my physiatrist first before doing anything to ensure it's the right step for me.

My question to you

Tyvense changes my life when it works, even at the 30mg dose, and I feel like something in my brain switches on but I barely take it to almost reserve it for the times I really need it. When that's days in a row, this can be a problem. Im wondering if anyone else has had this problem and or any idea to try to fix it. I've been talking with my consultant but they don't really know either.

Im finding it really hard to find any information on why this could be the case. If anyone has any studies or articles that might be helpful I would appreciate it.