Firstly its the role which is determined inside or out. Not you which is a very important distinction.
You can still have multiple clients (which makes you think you are outside) but for a particular assignment the role might be inside.
You don’t have to go umbrella. You can be paid PAYE as a contractor, or stay limited but pay employers NI. Speak to your accountant about that one.
Paying ubrella used to be worthwhile but you now cannot claim home to work travel so just as worthwhile going PAYE and saving your 25 quid a week the umb co charges.
IR35 has always been a law, the difference is who determines if you are inside or out. Used to be the contractor, now its the fee payer. Liability for any missing emp ni also falls to the fee payer so most companies are belt and braces approach.
Disclaimer-veteran of ir35 changes in public sector and also doing 90 determinations in the next few weeks so I hope I know what i am doing…