The consent and fetish systems were in discussions for an overhaul (for many of the same reasons as your post) while development was grinding to a halt due to a lack of time and interest. The GitHub version is slightly ahead of the itch.io version, though I'm not familiar with all the differences. The entrancement spell effect was included as a shortcut for both the consent and fetish systems, so if you are looking for an "always say yes" solution then that is it. Note that the enchantment spell effect is consumed by each attempt, so there is still a significant cost.
The consent system has a lot more complexity than may be apparent to the player and there are a lot of intertwined factors. Sexual consents can be influenced by lewdness, lust, and certain fetish opinions. If you aren't going to use the entrancement spell, then my recommendation would be to avoid trying to grind consent simply by asking every day as many of the consents require moderately high stats such as obedience and loyalty simply to get above zero chance of success. Unlike fetishes asking for consent does not improve your chances of success next time, so use a couple meet and sex interactions between attempts to gain consent. Also make sure to clear the rebellious effect of being captured or you probably won't get any consent. The Constants mod only provides options for chances of livestock consent.
The fetish opinion system is complex, buggy, and incomplete so it's difficult to give good advice for it. The alternative to the talk menu should be having the slave participate in the fetish during the sex interaction as the more times a slave is exposed to it each day the greater the chance of raising the fetish. However, this doesn't work for every fetish; some because of bugs and some because the design is lacking. The Constants mod provides access to some options, but these do not appear to be particularly useful at the moment.
To effectively alter/disable the systems, it appears that you could simply edit the values for "var fetish" and "var consentexp" in the program file ".../files/scripts/person/person.gd". It is a plain text file and the edits can be made with any text editor, though making a backup file is recommended. The fetish opinions can be changed from "none" to one of the following values:
"taboo", "dirty", "unacceptable", "uncertain", "acceptable", "enjoyable", "mindblowing"
The consent values can be changed from false to true.