You need to use other methods shown here, when testflight is full. Jailbreak is one type of method, but not necessary. Since scarlet and appcake aren't working now, you need to use paid service if you only have a phone. This community's focus is sonolus, and members might not be able to give an alternative method that suits you. You can ask different communities like sideloaded threads on Reddit.

For iOS: https://sonolus.com/download/Sonolus_0.5.10.ipa

For Android: https://sonolus.com/download/Sonolus_0.5.10.apk

iOS ipa is not signed, you can't run it by just opening the ipa file. You need to sideload.
If sonolus got updated, you can download it by changing the version code in the URL.

WE RECOMMEND TO SIDELOADING INSTEAD OF TESTFLIGHT, YOU CAN PLAY RIGHT NOW WITH SIDELOAD!

But here is the info, if you want to wait:
All test flight seats will expire after 3 months have passed. Current test flight seats started in late December to early January, and they will expire in late March/early April. We announce the test flight reset when it expires. Most recent test flight seats were filled in 1 week after it was reset. The maximum number of users of test flight is 10000. You'll need to rush when it gets reset.

Sonolus is free, you don't need to pay after the app is installed. The developer uses his/her own time to develop, so please donate via Patreon, if you feel this app is cool.

Sonolus is not a malicious app. I’ve been playing sonolus for half a year but no problems have arisen. And sonolus has over 50 million requests monthly combined from all servers. Many players have been playing sonolus without any issues. There are plenty of antivirus and software scanners available to help confirm the app’s authenticity. (ex. virus total) Discord can carry far more scam and malicious threats, so you should be fine installing Sonolus following one of the user guides.

But if you are unsure about safety, you can choose not to install.

Sonolus app doesn't include any server at first. You need to add servers from the #servers channel on sonolus discord. And you can get levels(charts) from the added server as well. The guide is here.

You MUST set the correct device audio offset to play. You can set them from settings at the title screen. The guide is here.

Open a level's info page (it will show up before going to the game screen), and tap settings. You'll see a Note Speed option at there. Title screen settings are global settings that are used in every game engine. And level info page settings are local settings that used in specific game engines. They have different options, so make sure you configure levels and note speed carefully. The guide is available at here.

For pjsekai refer to this:
https://wiki-en.purplepalette.net/create-charts

For bandori refer to this:
https://bestdori.com/community/charts/new

For bandori(bestdori): Yes, you can it on the website.
For pjsekai: No, unfortunately, you need a laptop for now.

You can search the chart from the website, if does not exists there, it is not charted yet. (But you can create a chart by your own hands.)

For pjsekai:
https://potato.purplepalette.net/fumen/list
For bandori:
https://bestdori.com/community/charts?c=Search

You can't change the difficulty of charts right now.
If you are new to rhythm game, you might think the charts are made automatically. but they aren't. All charts are hand-made by users who invest a lot their time to make one chart, and users don't make easy charts basically. (Since they are pro at the original game, and need more difficult charts)

Very unlikely.

Currenly, all game engines for sonolus are only developed by burrito.
But there are tutorials to make your own engine for sonolus.
Please feel free to make game engines by your own hands!!
We hope you can make a cool engine by your own hands!!

Please refer to the sonolus wiki, or you can ask questions to burrito.
At minimum, you need to have a laptop, and know what typescript is.
https://wiki.sonolus.com/sonolus.js-guide/