As I understand it, Swiss withholding taxes paid by CH-domiciled accumulating ETFs can be reclaimed as usual. Besides the very new ETF mentioned by @Tristan2001 another example for such an ETF is ICTax - UBS MSCI Switzerland IMI A-acc. On the ICTax page you can see ‘Gross return with VSt.’ and you should get 35% of that back after your tax declaration. That said, it might be difficult without an ICTax entry.
The SPI ESG Weighted index includes a lot more (smaller) companies than the SMI and based on the ESG rating some companies are excluded or over- or underweighted. The performance of the two indices might be relatively close but they might also diverge substantially over time.
If you’re happy with the ESG aspects, it looks like a reasonable ETF to cover Swiss stocks. The main downside I see is that it’s a very new ETF and so far it’s tiny (less than 12M in total assets). This may result in larger bid-offer spreads and there is a risk that the ETF will be liquidated if the volume remains small.
Why do you want an accumulating ETF? Tax-wise there is no real difference in Switzerland, are you taxed outside Switzerland? The only other advantage is that you don’t have to manually reinvest dividends, which may involve broker fees and/or stamp duty. However, if you’re regularly investing (part of) your savings into the ETF, it likely doesn’t matter much.