Need advice on UK visitor visa application.

Hi everyone,
I’m planning to apply for a UK visitor visa for a 15‑day trip to visit my elder sister during December. She will be sponsoring my trip for flights, accommodation (i will be staying at her place), food and shopping.

My background:
I work in a software firm with 6 years of experience. In my current company it’s been 1 year 2 months

1. I can provide leave approval letters, payslips. Is anything else is needed?

2. My parents live in India in my hometown (dependents to show ties).

Self apply vs agent: Is it better to apply on my own or go through an agent? I’ve seen mixed experiences on Reddit and it’s making me nervous.

Sponsorship concern: My sister is inviting me and covering the trip expenses. Will mentioning that she is sponsoring be seen as a red flag?

Visa rejection worries: Even with strong documentation, I see many rejections. Is it really down to luck, or if everything is stated clearly with proof, the chances of rejection are low?

Application form doubts I saw questions related to
the below and not sure about the answers

1. For mobile connectivity – should I get only an eSIM in London as the airtel roaming feels costly for just 15 days as there is a question for telephone number and to choose whether I will be using it in or outside of UK

2. Which address should I give as correspondence? I live in the city where I work and the duration of current location i am staying is for the past 1 year, my parents are in my hometown (I visit monthly).

3. Relationship status – I’m dating someone, but there’s no option for that. Should I just select “Single”?

4. Can I include my RD (Recurring Deposit) and ELSS investments under savings?

5. And majority of monthly spendings are rent for myself, loan for our house, and i send money to my parents, send money to elss and RD etc. These are some of the expenses that happens every month so there are a lot of debits. Aint it’s the same for many people. Is that an issue?

6. Should I add my RD and Elss for the question how much i spend every month, as it’s savings and the money is mine end of the day?

I have been dreaming about this trip for a long time now and I feel it’s not easy as I thought. Any guidance or experiences would be really helpful and mean a lot. Thanks in advance for your time!!