Yeah, it's not possible to book the train at this stage anyway. (However, you can count on the trip to take under 1h each way and not be very pricey.)

If you want to book a hotel in Heidelberg at this time, don't book anything which requires a pre-payment. Heidelberg's hospitality industry is hurting badly under covid. Some hotels, including the CP Heidelberg, have shut due to bankruptcy. Probably wise to avoid anything involving pre-payment for the overnight in FRA, either (for the same reason).

As for the flight into FRA, I would not have a problem with booking now for next May provided I find a good deal. You can rebook flexibly and the chance of LH going broke are fairly slim. Of course, it's entirely reasonable to hold off with committing to booking any part of the trip until February or March of next year.