I have a fussy 5 yo who also has a lot of food allergies so it's difficult to feed her.
At her most extreme her diet drops to pasta, chickpeas, frozen peas, olive oil, porridge, water biscuits, and grapes. This is her most safe list.
Her safe list is
Chicken, pork, chickpeas
Frozen peas, broad beans (if shelled), carrots (raw or lightly steamed), spinach (uncooked), sweetcorn, cucumber, broccoli (green tenderstem only)
Pasta, rice, noodles
Apples, cucumber, oranges, grapes
Porridge, toast, Weetabix, rice Krispies
Bread sticks, water biscuits, crackers
Never likes
Mushrooms, parsnips, turkey leg meat, butternut squash, sweet potato
She still has 3 Easter eggs left from Easter, a sweet cone from a birthday party in December and Christmas sweets haven't been opened.
Pick your battles, pay attention to good days /bad days. On good days try to push and encourage comfort zones to stretch a little on bad days stick with reliable foods. We go back to the most safe list when we need and then start trying to get some more of the safe list in. If we do that we look at getting other things similar to the safe list to add more. At 3.5 - 4 we spent more time in the "most safe" space but now at 5.5 we spend more time in "safe zone"
Sauce and gravy is a bit hit and miss. We have a small milk jug and we put her sauce and gravy in that and she has a choice about how much she puts on. Arrange at the table meals work well. We do noodle broths. We dish up noodles and meat and she adds the broth and finely baton veg
We are under paeds and dieticians and some times she grows regardless other times her growth stalls. It's stalled at the moment and she's gained 1kg in 18 months and dropped nearly 2 percentiles. At the moment volume of food is a bigger issue than range of foods. I tried adding chocolate spread and peanut butter to her crackers to up the calories but she took a small bite and didn't touch the rest. She has this multivitamin as well as an omega 3 gummy (no fish due to allergies)