Confused About BIR Registration, Employed to Freelance

Hi everyone! I'm hoping someone can help me understand my tax situation because I'm honestly getting overwhelmed. I've been reading a lot and I still have so many questions.

Here's my situation:

• I was employed from January until April 3, 2026.
• My employer already gave me my Form 2316. My year-to-date compensation was around ₱260k, plus around ₱41k in final pay (unused leave conversion)so the total was roughly ₱300k. They already computed and withheld my taxes and deducted the  ₱250k. The taxes was about  ₱7600 since it was 15% right because I was under a company.
• I became a freelancer after that. I didn't work for the rest of April and started with my new client on May 5, 2026.
• Since May, I've been earning around ₱60k–65k/month.
• I haven't registered with the BIR yet, but I plan to do it this coming Monday.

Here are the things I'm confused about:

1. Since I was an employee first and now I'm freelancing, I'm considered a mixed-income earner for 2026 right?
2. How do I compute my taxes for the April to June quarter? Do I only include my freelance income from May and June or does my previous employment income affect the computation?
3. What forms do I need to file after I register please?
4. Around August or September, my rate will increase from $6/hour to $8/hour, so my monthly income will go up. Does that simply affect the next quarter's computation or is there anything special I need to do?
5. Since I'm registering late, what documents or requirements should I bring to the BIR? Or what are the important documents I need to bring that may not be in the website, like any tips maybe. 
6. Can I also pay any taxes due on the same day?
7. I've read that I will need to issue official receipts from the booklet they will provide once I'm registered. Since I've been working since May, can I issue invoices for the services I already rendered? Do I need to send those invoices to my client, or are they mainly for BIR record-keeping?
8. For those who have gone through this, how does the BIR verify invoices? Do they actually ask for them during filing, or do you just keep them in case of an audit?

One reason I'm trying to get everything in order is because I plan to apply for a UK visa in January 2027 and I wpuld like my tax records to show that I've been working as a freelancer since May 2026. It helps with my rootedness and a hugher chance of getting approved.

I know I probably should have registered earlier but I'm trying to fix everything now and do it properly. I would really appreciate any advice from freelancers, accountants, or anyone who's been in a similar situation. Thank you so much! This was sooo long