Entering if you have a visa shouldn't be an issue, just be aware that there are are limited number of countries with flights in and out of Russia.
Maybe you are aware, but a few notes anyway:
- random people don't speak English, translation apps are easy enough and they aren't being impatient
- your existing banks and cards do not work anywhere, just pretend they don't exist and bring enough euros or USD (cash) with you for your entire trip
- moscow and saint petersburg are fairly cashless, many bars, stores, restaurants assume you will pay with card or other electronic payment method. They may switch to cash I'd you ask
- getting a local bank account is very easy to do online and a person will hand deliver your new bank card, this allows youe to deposit cash into ATM and then pay for things using a card and RUB... It just requires that you have a local sim card / local phone number that nobody else is already registered to a bank account
- getting a local sim card / phone number is basically impossible unless you have a local friend who lends you their spare sim card that they purchased in their name
- if you can solve local sim card and banking, then delivery and transport apps become available and make your life much easier, the apps are only partially treanslaed to English though so be prepared to translate screenshots from time to time.
- getting data is easy enough using a roaming esim that specifically supports russia, likely one issued in Belarus, useful for data but not for registering for services
- most of the things you want to do on your phone are blocked and will not work with out a VPN to tunnel out of Russia
- most VPNs are blocked using very effective deep packet inspection at the internet service provider, this means that the VPN you already have probably doesn't work and will require some technical messing around
After 6 weeks in Saint Petersburg, that's basically the list of things I believe someone would want to know before they got here.