Ah I see...if you're looking at root stretches that's why you're getting darker areas underneath. It needs to be staggered in order to blend it through. The head is a round object and because it's round, areas will be stretched down further to create balance and depth. You'll especially notice this at the nape and diagonally behind the ears and will be much darker through the area of where you occipital bone is. This can look off if you're just focusing on those sections but it helps with the over colour results. These types of colours also always look better with a wave/curl/texture as they are meant to be more of a 3d technique rather than a mirror finish.
It sounds like to me, you probably want a more tradional foil highlight. By using a root colour you're adding another layer of synthetic colour which needs to be broken through when you have your next service. This over time can cause patchy results and warmer areas through out. Even with the best of colour technicians. It's brilliant on the first couple of appts but then gets very hard to balance there after due to the nature of the technique and the limitations of the product.
It sounds like of you're looking to lengthen your time between appts and have a more subtle regrowth, I'd recommend a root melt with a toner a shade darker than your lengths and ends. It will leave less of a build up but won't last as long.
You could also try having a more traditional full head but instead of a tradional weave, have a combination of teasy lites and super fine baby lites put through. This is much more time intensive for the stylist and may incur a surcharge. The results are beautiful, bang on trend with a softer regrowth and gorgeous blend through. And overall brighter blonde finish. Go for a wow at the front and a slightly lighter contour or money piece put in. It doesn't need to be thick just a couple of foils depth to add some brighter ribbons at your hairline and to show off eyes and cheek bones. This will address the regrowth directly from the root but still keeps it soft. If you have been having darker colours put through at the root, just have the bleach foils done as per above. Just using bleach sounds scary but it can create so many colour combinations with contrast and depth as well.
These concepts are very, very much on trend now and yes you're correct in the last 5 years especially since covid a lower maintenance, rootier finish has been the main trend for highlights/balayage. There's also nothing wrong with a beautiful set of classic back to back blonde foils.
These looks are hard to achieve and you do need to see someone who is strong in colour and specialises in being light handed and subtle. Part of the reason we see some bad balayage and darker root looks is because the hair hasn't been highlighted appropriately for your features. Ie what are you trying to enhance? Cheek bones, neck line, shoulders, eyes etc. That's the starting point for building these colours correctly.
And, as far as price goes...I'd expect a minimum of £260 -£300 plus. They are time consuming, heavy use of product and super skilled/technical finishes. And, it sounds like you're having a full combination of differing techniques used.
Sorry about the essay! As you can see it's something I'm passionate about and work with on a day to day basis.