I either remove and replace the OEM insoles, or augment them with a cushion or gel heel insert.
Over the years, I have only had problems when I used a slightly thicker replacement insole, and they reduced the toe-box volume too much on those boots. I had to replace the insoles with a set that had a thinner forefoot. The thicker insoles were not a problem when I used them in boots with a slightly larger toe-box volume.
There are several different styles, some little more than a cloth covered strip of thin rubber padding. At that end of the market, they offer little protection and support to the heel. I prefer those which have a wedge and cup at the heel to give more protection and support. Most of these also offer some arch support, and if you don't need that, a heel insert might be sufficient.
I have tried gel inserts, and use them occasionally, but prefer the denser foam types for longer walks.
Regards,