I totally understand what you said and can appreciate the mentality of needing something to aim for.
What I would suggest is, that instead of having the meal once a week, you could still do the once a month thing, but then you could go out somewhere to eat instead. This way there is no temptation, when you buy stuff that you know hasn't helped your eating habits in the past, to buy too much and have too much at home, where it might weedle its way back into your lifestyle again, causing minor relapses in discipline.
Eating the planned meal away from the house, means you are eating cheat food, but you are not in a situation where you are potentially overbuying the food and keeping more than is necessary at home and possibly having too much in a week, because to overeat at a restaurant for example, would require you to make a more conscious decision about the cheat, rather than going to a supermarket, being bombarded with convenience cheats and perhaps buying too many of them, where they can be in the house and close enough to access.
The once a month thing also means it's more rewarding, by saying, yes I went 4 weeks without making mistakes and also, it become more affordable, than trying to find the money for a meal out once a week.
Obviously you know yourself best however

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You should be proud of yourself, for making this aim of yours actually come good, but like many of these things, I.E. people recovering from alcohol problems, abstinence can be achieved for long periods, only for relapsing in a serious way to occur with many people and I'm not criticisng your life choices, but slight concern is, if you tried to have cheat food once a week, you might at some point find it's past influences on you creep back in, whereas eating it once a month, away from the house, means it can't, because you eat it, but then you go home to a cheat free environment.
Whatever you do do though, I will support the decision 10,000%, because I want you to achieve your aims in as happy and as productive a manner as possible and I always try to comment constructively and with someone like you absolutely coming first and I will say that I am really proud of you for achieving your aim, because I can appreciate how much these little things mean to you and what they represent and I'm sure you will understand the ethos of what I am trying to express

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Keep going and WELL DONE.