Retouching on an already built website's front-end pages (3-5 pages) and back-end (price changes to be made by admins) on admin panel

Our website [login to view URL] is created last year using ASP.NET MVC Core 1.1.2 and working fine at the moment. Since the person who was working with us has left at the moment, I would like to resolve some minor issues and change front-end without losing any of the back-end features. We used a template for the front-end of our website, [login to view URL] - We are unhappy about the home page sliders. Size changes in different platforms such as iphone or PC make the image text unable to see. I want to change the front page so that it looks more stylish. Sliders are not necessary, we can have a different style for the front page. - There are some pages where text is so small to see and they are in the client's profile. I would like to change the style there to make things more visible and stylish. - The database is not fully integrated with the admin panel. I would like to be able to make any changes in the database in a secure manner (not just with one click). - Our products (travel packages) have a price change when 5 people buy each of these packages. The formula for this change is done using some back-end features. I would like to be able to change this formula at front end as well. - The website includes a currency rate which affects the price of a product. The currency rate is held in the database and is only changed manually. I would like to fetch this rate from a live currency exchange site and multiply it by a value to affect my product's price. - Our site includes a representative login option where each representative can see his/her clients who purchased a product choosing the representative's name from a drop-down menu. The representatives can see his/her earnings from their login page but the earning rates are decided in the back-end and it doesn't have an option where we can delete the paid amount from the earnings. I would like to be able to update the earning rate and the payments made on this profile. - I would like to be able to add an extra service on a product (such as insurance) and be able to remove the existing ones if I want to (there are already existing extra services on a product such as hometown transfer and visa consultancy). - The client log-in is made possible using an activation email. The activation email arrives too late in the client's mailbox for an unknown reason. This could be due to the smtp server of the mail website we are using (yandex), I want to resolve this by using another email account for activation. We already use g-suite, so we could use an email address from gsuite which already has our website's url in the end. - I want to add extra columns in some of the tables in the already existing database. This extra column can be nullable and be NULL for the already existing rows. Since some of these columns will be for web-forms, the forms in the front-end will also have to change accordingly. - We use an SMS client to send some automated SMS to the phone numbers of our specific clients according to the product types they bought and the time they bought it. This was actually working until recently, now it doesn't work. I want to be able to resolve this issue and maybe add some extra SMS features, too. - Online payment system works well, but it brings up a blank page at the end of the payment. If client refreshes this blank page, the result of the payment can be seen, but it stays blank otherwise. We are unsure of the reason as to why this happens. We are using 3D secure payment system and are in cooperation with a bank in our area. If you can resolve these minor issues and make small changes on our website, I am willing to work with you in the near future to add more services on our website.