Hey... I am training a few months and been trying out things a lot.

My thoughts on this:
I do one day (strength) focused in rep
range 4-7 and one day in rep range 9-12.

Now my big issue with this is basically I can't do 4-7 or 9-12 reps in 3 sets.

If I choose a weight that hits for example 4 reps, my second set will only be 3 reps and my third set 2 reps.
Or if I choose a weight I can hit the 9 reps, my second set will fall to 7 or 8 reps and my third set to 5 or 6.

So if I don't ADJUST my weight I chose for the first set my reps are ALWAYS going down duo to fatigue.

This brings me to the next issue.
I will choose rep range 9-12 as an example.
My first set I do lets say 12 reps. My next set 10 and my third set 7 reps.

The next time I still can't increase the weight even though I did 12/12 reps on my first set, cause both other sets are not 12/12.

But by not increasing weight I can now do 14 or 15 reps with the same weight on Set 1 because I got "stronger". But again, now that I did more reps on Set 1 I still can't do more reps on Set 2 and 3 cause of fatigue from Set 1.

How can I ever get 7/7 or 12/12 if Set 1 is pre fatigueing me?

Choose a lighter weight and do 12/12 on set 1 even though I could do 20+ more reps or what? That makes no sense. But only with a lighter weight I can complete 3 sets of 12/12 or 7/7.

I dont understand how this should work. Choose a weight that I can do 15 reps safely but still only do 12 so I can do set 2 with 12 reps and set 3 with 11 reps and then up the weight?

That would mean I have to do set 1 with 3-4 reps in reserve, so I can do atleast 12 reps in set 2 and a minimum of 10 reps on set 3.

My honest opinion on this after trying it for a few months.
I think I am better off blasting my self on Set 1 with 1-2 reps in reserve instead of trying to hit 12 reps in all three sets. Then I use the same weight (if I think I had 1-3 reps in reserve) and AMRAP Set 2 with 2.5 kg less and not do a third set at all.

It feels like, or atleast that is what I think I am doing: trying to prevent fatigue in set 1 so I have enough energy for set 3 and hit my 12 reps.

Im not sure if that is good... it feels like Set 1 is totally wasted cause I cant go "all in" duo to being afraid of not getting the reps I want in set 2 and 3.

Today I dumbbell bench pressed 10 reps with 25 kg dumbbells.
I quit at 10 reps cause I knew that if I did 3-4 more I wouldn't be able to do 8 reps in Set 2 not even 6 reps in set 3. So I did 1. Set 10 reps, 2. set I went for 12 reps anyways and my 3. set failed again at around 7 reps. And again I wasnt able to stay in my target rep range of 9-12 cause of doing 12 reps on set 2 instead of 10 reps.

I honestly have no friggin idea how people can do 3 sets of "12" and then increase weight after doing 3x12 back to 3x8 until they are at 3x12 again. Because either I can do 1x16-20, 1x10-14 and then 1x 6-9 but not all three at 12 reps. Heck I can't even choose a single weight for 3x8 cause if I choose a weight that hits the "8 reps" directly I wont ever be able to do 8 reps with the same weight after 3 minutes rest again on set 2 yet alone set 3. It always goes DOWN. 12 -> 10 -> 8... or 7 reps -> 5 reps -> 3 reps... but never consistent reps. Its always set 1 good, set 2 worse, set 3 very bad.

Edit:
As a beginner in weightlifting this is totally complicated for me to understand.

I thought Id give rep schemes another try today but it feels like my new approach of doing drop sets feels better to me. I am just choosing a weight with which I can do 12-14 reps, but quit at 12 reps and then use that same weight and work my way down to 8 reps.
So my first set is usually 12 or 13 reps, depending on how I feel and assuming I could do 1 or 2 more but still stop.
Then I do my second set with that same weight and AMRAP that thing (usually around 10-11 reps) and then I finish my third set with a bit of lower weight but also aim for 10-11 reps again.

What I also tried is just increasing weight after my first and second set are both around 10-12 reps even though my third set is still only at 9 reps.
Just increase anyways no matter what the litature says 12/12/12. Because by the time I hit 12 reps on my third set with a specific weight I am basically doing 20 reps with that weight on Set 1 and 15 Reps on Set 2.

But honestly for me as a beginner it is way easier picking a weight that I know I can lift around 13 times, give my best and stop the set once I think I can't lift 2 more times.
Or basically just increase weight as soon as I hit 9-10 reps on set 3 instead of waiting til 12.