I use Riverford too, substitutions were relatively rare pre-Covid but seem to have increased a bit since so I would give it a bit longer if that's an issue for you though sympathise with the annoyance of your careful meal plan being thrown into disarray by a sub... I love the quality and the ethics of the company personally and find the customer service really really good.
I find the veg box system works great with meal planning, the principles for meal planning with a veg box are basically just normal meal planning with an extra factor thrown in, you still rely on making and sticking to a plan, knowing what you already have in the fridge, freezer and cupboards, organising your shopping around the plan and your existing stock, and eating leftovers for lunch or freezing for another time.
I find out what is in the next week's box on a Friday (and again pre-Covid you could chop and change between boxes, now a bit more limited but I would expect to go back to 'normal' at some point) and I make the week's meal plan on a Sunday. I prioritise using up anything we have leftover from the previous week's box, then meals which use things in the next week's box, then anything we simply fancy. So e.g. we might have some leftover carrots and onion, and I know we are getting cabbage, peppers, spinach, squash, broccoli and potatos in the veg box, so I'll plan sausage and mash with carrots, brocoli and onion gravy on Monday, then Tuesday will be stir fry with cabbage, pepper and broccoli, then Wednesday will be curry with squash and spinach and so on. Some veg are more challenging to use (kohlrabi anyone?) whereas others fit right in with our family favourites but that's where the excellent website and the little cards they put in the box with recipe suggestions come into their own.
As I make the meal plan, I note against each meal any ingredients needed which we don't already have in, e.g. I'd check the stock of dried egg noodles and soy sauce for the stir fry, the spices, chutneys and rice for the curry, and make up a shopping list of anything missing.
I bulk buy dry ingredients once a month and do a supermarket trip once a week for anything that is needed that week that we can't get from Riverford (Riverford do eggs milk and bread). My ambition is to reduce the need for the supermarket right now but lockdown has made that slightly trickier!
If it gets to the weekend and there is some stray veg floating around (e.g. we didn't use all the potatos for the mash, extra carrots or whatever) I make a big pan of 'miscellaneous' (sometimes euphemistically titled 'garden veg') soup which does for lunches the following week or can go into the freezer. Also, if there's some veg I just can't or don't want to use at all (to be fair we do all occasionally get a bit tired of potato), most things can be blanched and frozen, or fruit can be chopped and frozen raw for smoothies etc. Every so often we cancel the veg box for that week (particularly end of the month if we are a bit poor) and we just eat the odds and ends which have made their way into the freezer.
One tip I might suggest is maybe when you first start order a slightly smaller box than you think you might need, then if you find you are easily using it all and even needing to buy more from supermarket, go up a size, but although in theory pre-lockdown we could use a 'medium' box for our size of family we managed fine on the 'small' even though we eat loads of veg, due to some meals being eaten out etc, if we got a medium some would have to be frozen or carried over to next week. In lockdown we are of course eating everything at home so bit different but just find the right level for your family...