I might catch some heat for this... Steeping is hogwash... Do not steep... Hot water bath, letting things sit with their caps off, so on and so forth... all that does actually is help destroy the flavor molecules. A good bottle of juice is good straight out the gate. My mixes are VG and the flavoring and that's it. I shake the bottle vigorously counting to 100, sometimes 200. If it doesn't taste good after that, then it's a fail IMO.
Make test batches in 3 or 4 ml amounts so you do not waste much. Here is what I do when I want to figure out a single flavor:
Bottle 1-1 drop per ml, bottle 2-2 drops per ml, bottle 3-4 drops per ml, bottle 4-6 drops per ml, etc.
*Don't forget to measure your drops so you can determine the % afterward.
Now test using an RDA. Better yet (what I do), a different RDA for every different bottle %. Keep coffee beans and water at hand to help cleanse nose & palate between tastings. Next, let the bottles sit (with their lids ON) for a few days and then try them again. Whichever one still has enough flavor is your winner. Record your favorite. It helps to have a partner so you can do all the tasting blind. You only find out which % you liked after your partner tallies the votes.
Also, I've made many clone recipes and DIY recipes posted on the ole' interwebs... most of them suck and are weak.
Ethyl Maltol mutes flavors, do not waste your time. It helps when you want to add... hmm.. a "darkness" I'd say - but at the cost of muting the profile quite a bit actually.
One good thing about vaping weak juice is that your palate will adjust and you will get used to it. I vaped super weak juice for 6 months because I did not know any better. Ignorance is bliss in this case. I then started trying things like Bobas and Vapor Chef = my juice tasted super weak compared to those. I then went back to the drawing board and now consistently hit strengths that I am happy with. It has taken a lot of work and wasted time.
Do not expect to make strong tasting juice straight away - especially if you're relying mostly on the WWW for your instructions. On the other hand, do not give up, do not let yourself become frustrated. You will find what works for you, trial and error is your best friend.
Here is a recipe that changed the way I thought about DIY juice. It isn't mine but it helped me quite a bit.
Peach Lemonade-
Flavor Art White Peach 13.5%
TFA Lemon 16.5%
Lorann Tart & Sour 3%
*Note - do not replace the ingredients with other flavor makers versions - it will not work. Also, these are very fluid flavorings so in this case 33% flavor really dilutes the juice. You will not win any cloud competitions with this one

But it does taste like really good peach lemonade and at 33% you can rest assured it's a strong flavor.
PS - If I were doing a Strawberry Lemonade I would start with:
TFA Lemon 16.5%
CAP Strawberry Cream 3.5% (that's right, I said Strawberry CREAM)
CAP Sweet Strawberry 10%
Lorann Tart & Sour 3%
Hmmmm... maybe I'll make some right now.
Any way - hope this has been helpful in some way.