Hopefully this reply isn't too late. By way of clarification, I'm a senior adviser for a large IFA firm, I advice on this exact situation for 100's of clients a year.
I thought I would clarify a couple of things.
EFRBS legislation is extremely unlikely to apply but you could check. It's an unapproved pension scheme so your employer is very unlikely to use one.
Please do not consider EIS or SEIS schemes unless you are an experienced investor with a high tolerance to risk. They have very attractive tax advantages only because they are 'risky'. The tax advantages are there to encourage investors that have the aptitude, knowledge and experience to accept the risks involved. Most average investors do not.
Typically you can take the £30k as a tax free payment and in 'most' circumstances making an additional payment into your works DC scheme is sensible due to the tax advantages. If as you say, you firm offers salary sacrifice they pay the additional payment £35k into the DC pot so you have no need to claim the tax back (as it won't have been deducted AND you also bypass the National Insurance contribution). From a tax perspective this can be very attractive however, this is subject to both Annual Allowance (AA) and Lifetime Allowance (LTA) issues.
AA - You can make a maximum annual pension contribution of £40k as long as you earn at least £40k. Essentially, if you earn £30k you maximum gross contribution (employer, employee or third party) would have an upper limit of £30k. However, you are able to utilise 'unused' AA from the previous 3 tax years if you earn above £40k. In this tax year your relevant earnings will be your salary (from April 2020 to July) + the payment above the tax free amount (£30k) so £35k meaning you could contribute (via salary sacrifice) as much as £75,000 providing you have unused AA from the previous 3 years, this is called 'carry-forward'.
LTA - The maximum value of your combined pension pots (including DC and DB (Defined Benefit)) has a maximum limit of £1,073,100 (2020/21) meaning that whilst you can contribute more and the pots can still grow above it you will have a lifetime allowance change at some point (latest of age 75) which will negate the benefit to the tax relief on the way in.
Pensions are complex, please seek advice from an IFA, this should only be constituted as guidance as without knowing the full financial picture nobody can determine the suitability. The fact you mention AVCs (Additional Voluntary Contributions) would sometimes indicate having a separate Defined Benefit pension so it's important to understand how a potential DB pension has increased in value over the last year as this will utilise some of your AA.