Am I making a mistake by topping up during boil??

So I've done about 8 partial mashes at this point and I'm wondering if I should be doing this? The beers are turning out really good but I don't know if this has an impact.

I have a 5 gallon kettle and with most recipes by the time I combine the sparge water with the main kettle, I'm somewhere around 4 gallons in the brewpot.

After it's started boiling and I've added my extract and hops, I top up the kettle pretty darn close to the top, so that I'm boiling almost 5 gallons. I leave just enough room so I don't get a boil-over.

By the time I've cooled the wort and moved it to the fermenter, I need very little cool top off water. Does this make the beer more solid because you've boiled most of it together??

Is this what I should be doing?

Consistency of technique is probably the most important part of homebrewing. If your recipes are designed for a 5 gallon boil, then keeping it close is a very good idea. Any makeup water during the boil should be hot to keep the boil going. Your beer is turning out well, so what you are doing is right for you and your system.

I just hope you're getting a really good rolling boiling with the wort made from your minimash. Get that done for a while before you add your extract and top up to the rim of the pot. Certainly you can't get a good rolling boil with the pot that full otherwise you WOULD boil over.