Modifying a WordPress Plugin

Introduction
　I’m creating a price estimator for a client’s website using a WordPress Plugin.
　
　The plugin works great for how I originally used it, but now the client wants to change things up
　completely!
　
　Instead of developing something from scratch, I feel it will be easier and more cost effective to modify
　the behavior of the plugin to achieve the visual style the client wants to achieve.
　
　
The Plugin
　Here’s the plugin: https://codecanyon.net/item/wp-cost-estimation-payment-forms-builder/7818230
　It’s used to create cost estimators.
　I can provide the plugin files if needed.
　
　
The Modifications
　There’s two things I want to implement now:
　
　Change: Modifying the Input Field Spinner Arrows.
　Example: https://stackoverflow.com/questions/45396280/customizing-increment-arrows-on-input-of-type-number-using-css
　Note: The plus and minus signs would be nice, or even just any version of the buttons being on either
　side of the field and/or larger. Honestly, anything is better than what I currently have, which are tiny
　spinner arrows that are annoying to find, mouse over, and press. This is the main and most important
　change.
　
　Change: Collapsible Section Functionality
　Example: https://www.w3schools.com/howto/howto_js_collapsible.asp
　Note: The client offers a variety of home services. I want to add the name of the home service as a
　header on the collapsible field, so when the name of the service is clicked, the other content is shown
　and the user can then make selections from that. (The plugin has the ability to hide content, but it
　removes that content from the calculations, which isn’t acceptable.)
　
　
Modification Method
　It would be nice to have the changes implemented as added features to the plugin, useable through the
　back end. If possible, it would be nice to have it setup in a way that remains when the plugin is updated,
　as it’s frequently updated. (I’ve read a bit about how to implement this, but I don’t have enough
　experience in that regard to know if it’s actually possible or recommended. The most consistent solution
　I saw was to simply inspect the updated plugin for differences and add the code again. Though…now
　that I’m thinking about it…that doesn’t sound like a realistic thing to do without the plugin frequently
　breaking…)
　
　The other option is the designated sections within the plugin to modify the individual forms using
　Javascript and CSS. The code in these sections should remain even after update. The individual elements
　like the input fields can be given CSS classes through the form builder, but the sections of the form don’t
　have such an option. There is a Designer tool that can be used to select and find the names of the
　elements and their classes and such however, if that can be modified through Javascript.
　
　Here’s a video showing the back end: https://drive.google.com/file/d/1UP_2Jbj8ld3_lzIy_B1N6qCc5Q9_-czW/view?usp=sharing
　
　
Deliverables

　Modified plugin files
　OR
　Stand alone code to modify plugin behavior.
　
　
I would greatly appreciate any help on this.
And I have another bigger project with the plugin after this too!
　
–Thanks – and have an awesome day!