I love rapini & can't wait to eat some of the crop currently growing but it's taking forever to show any signs of flowering. I'm just seeing lots of leaves. Should I fertilize with a bloom booster? Or just wait patiently for the blooms that will eventually appear?

I wonder if it's the time of year? They may be waiting for Christmas. I mean, for the day length to start getting longer. Although I suppose your day length doesn't change much.
I haven't actually grown rapini, but I understand there are some varieties grown only for leaf, others to cut as soon as buds form, and that they likely have different DTM's. Is this one you've grown before? I would think that harvesting some before they flower would also be good to eat, if you can't wait for the buds.
I grow Yu Choy Sum, which is in the same B rapa family and also harvested when it flowers. It is really a fast crop, and I've never had to encourage them to flower by fertilizing. I plant those 4-6 inches apart in pots during the winter, and they flower and get harvested as a medium sized plant after about 40 days. Possibly the close spacing encourages them to go ahead and flower early? That is definitely a way to get brassicas to bloom earlier, by tight spacing.
And I also grow Kai Lan/ Gai Lan which is B oleraceae like broccoli, and that takes about twice as long to be ready for harvest, being a slow growing but much more robust plant that makes big stems and finally big flowering shoots. They take a long while regardless.
Either way, I only fertilize at transplant time so I can't comment about the bloom booster approach.

I also fertilize only at planting time for my winter crops. I recently ate all my Yu Choy Sum, they were flowering. I only had a couple of plants so maybe that's why I'm impatient for the rapini to finish up lol. Anyways I sowed more Choy Sum last week, hopefully I'll be able to get more soon.
Meanwhile I have tons of lettuces, but who wants salad during the cold days? Not me. I grow them to give away, plus they're so easy. I just throw seeds around and they'll sprout after a rain. Sadly none of the arugula seeds sprouted this time, so I did another round. Fingers crossed cuz I love this vegetable too.
Thanks @Bower for your encouraging words. Yes I'll just wait for those rapini.

Funny thing about lettuce in cold weather, eh. My mom says the same thing. I started a load of romaine anyway because they're charging $10 a head for it here. I still like the occasional salad but only as a side for something good and hot!
Arugula, parsley and cilantro are all really great to stir into a dish of hot curry when served. They can take the wilting and still be bursting with fresh flavor. I only regret that arugula bolts so quickly. We'll only get a couple of cuts before it's done. Been thinking about trying the perennial type again. They are stronger flavored but for a winter dish it might be just fine to use it as an accent/herb. Good luck with your seeding!

Lots to do with lettuces besides just salads, which I myself like all year round. I will use lettuce leaves to steam fresh peas in, I will wrap a lettuce leaf around a steamed or fried chinese sort of dumpling and eat it up like that. Use a leaf as a taco shell or sandwich bread. Add to smoothies even. I have used lettuce leaves instead of cabbage leaves to make rolls to steam, stuffed with meats and veggies.

Would love to know your favorite lettuces, Danny.
The Sherwood romaine was really fantastic this winter. It took a bit longer than I expected, and they didn't get as large as I hoped, but what a great flavor. I will grow this one again and again.
I imagine something with a large juicy and frilly leaf would make an amazing wrap as you described....

I really like the choi sum varieties quite a bit too, and so quick /easy to grow. We always have eaten broccoli and cauliflower leaves that are tender as they are tasty too. The choi or the leaves above, lightly steamed and in a meat sandwich such as a salty ham or pastrami, yum and adds a nice taste and crunch.

I don't a favorite lettuce per see, but a favorite type is either an iceberg or romaine type. Love the crunch and tastes, refreshing.I have way too much lettuce seeds I am sure, but grow it both as a cut and come again and as full heads depending on the time of year and how darn hot it is. I like lettuce sandwiches, plain white bread ( toasted is good too, but cool it before buttering it) buttered and about 3/4 inch thick layer or hunk of crispy crunchy lettuce with salt and pepper -- refreshing when it is hot out yet enough to carry you to the next meal. Of course, always a delight to add bacon and homegrown tomatoes to that too.

I've been eating leaves off my Purple Sprouting Broccoli/ Purple Sprouting Kale this winter - just amazing crunch in the stem and the leaf is fine raw as well as cooked if you slice it up thin. Lovely stuff while waiting for the 'spears'.
Iceberg and romaine are my favorites too. Red Iceberg is awesome.