Firstly I only have a lay interest in this, I read a lot of abstracts from scientific papers, read blogs and watch Youtube... but content and interviews with the kind of people with relevant PhDs, and filmed lectures from conferences by leading vitamin D researchers etc, ie NOT wellness vlogger Gwyneth Paltrow types. Evidence-based scientific studies and up to date research. With that caveat in mind, what I'm doing is passing on the info from those experts and backing up info with links to that content and research papers.
For you, I'd say make sure you're taking 4,000 iu of vit D per day (till at least the spring equinox). If you want to take a bit more, read up on it first, and calculate how much you need with the calculator on the Grass Roots Health page, or the DMinder app for smart phones. Also talk to your doctor if you have a chronic condition, especially where high calcium is a risk such as kidney disease/stones/arteriosclerosis etc. Also postal vitamin D testing available from NHS Sandwell Trust, Thriva Medichecks etc for around £30.
www.grassrootshealth.net/project/dcalculator/
4000 iu/day is the upper recommended amount from the NHS that is considered fully safe for (almost) everyone. Above 4000, there is a very gradual increased risk of elevated calcium, but that can be mitigated by taking D3 in combination with K2 and magnesium... they form a trinity of co-factors, so that's best practice anyway.
Remember it's not so much how much you take per day which can be toxic, it's the blood serum levels reached which are considered toxic or safe. 75 nmol + is adequate, 100 - 150 is optimum, and above 150 is considered surplus but safe. It's generally unnecessary to go over 150. Over 250 becomes properly toxic.
If you're concerned about taking more than 3000, take that... you're still doing heaps better than most of the population, who only get a micro-speck of vit D from the odd fishfinger or generic multivitamin pill.
As for your diet, start with cutting out sugary drinks, sweetened tea & coffee and soft drinks. Only have a couple of glasses of booze at the weekend (dry wine/spirits... beer is "liquid toast").
Aim to eat real food, ie plants that came out of the ground without factory processing, and the animals that ate unprocessed plants that came out of the ground. ie grass fed meat, fish, eggs, unsweetened dairy products, vegetables, moderate amounts of unprocessed fruit etc. Things like frozen and tinned vegetables and fish are fine if in their intact state... ie not processed further with a dozen additives into hyper-processed gunk. Juices and smoothies are as much a sugar bomb as a huge slab of cake. Fruit needs to be in the context of its natural moist fibre matrix... once you blend it, juice it, dry it etc, then it effectively becomes pure sucrose/fructose. Minimal added sugar, refined flours, margarines and seed oils etc, unless it's an occasional holiday/party/celebration etc. Watch Prof Robert Lustig's talk on youtube "The Bitter Truth" (he's an endocrinology expert) or read his book Fat Chance. David Gillespie's books on sugar are also great "sweet poison" is very scientific on the metabolic effects of sugar on the body, and the follow up one is about how to apply that to give up sugar.