M&S frozen roasted veg - another game-changer that i use with my precooked frozen onion cubes...
I'm vegan and I do the same thing with cashew cream which I use instead of dairy for 'cheese' sauces, bechamels etc.
Also M&S pizza bases which I freeze along with mozzarella and roasted peppers etc.
Always make 2 lasagnas as it's such a bloody faff
Amazon coffee pods on 'subscribe and save'
I'm lucky that I have quite a lot of storage space and usually enough cash to buy two or three of the things that are a pain to get hold of in some way, either because they're only available from one shop or they're much cheaper in one shop.
I use Mac/iphone Notes for all the info I need on a regular basis - just cut and paste it into the email or whatever. Especially good for children's NHS numbers, energy/insurance account numbers etc.
Also Gmail - I have made email templates for the five or six regular queries that I get, it saves me huge amounts of time. Especially, weirdly, for when I have to say No to something - I found that crafting an email which says you are the cheekiest of CFs and here's why, without being rude, used to take me aaaages and I would agonize over it.
Also in Google Drive - you can set a folder as a favourite and it will come up in Finder if you're on a Mac, especially good for things like my rates that I often have to attach to emails.
I also have spent a ridiculous amount of time making my iphone homescreen work for me - I use the large widgets for things like Reminders so when I pick up my phone the things I need to do are right there. I tried various 'productivity' apps but actually iOs/Mac Reminders ended up being the simplest/cheapest/best and I always add a date so I get an alert. I've got an elaborate workflow to get Alexa to add something to Mac Reminders so I can shout at her in the kitchen and it will appear on my phone. Even though it was a PITA to set all this up it really works for me and keeps me calm <ADHD face>.